The universe is hiding something. In fact, it is hiding a lot. Everything we experience on Earth, the stars and galaxies we see in the cosmos—all the “normal” matter and energy that we understand—make up only 5% of the known universe. The other 95% is made up of two mysterious components: “dark matter” and “dark energy.” We can’t see them, but we know they’re there. And what’s more—these two shadowy ingredients are locked in an epic battle to control the very fate of the universe. Now, scientists are trying to shed light on the so-called “dark sector” as the latest generation of detectors rev up, and powerful telescopes peer deeper into space than ever before to observe how it behaves. Will the discoveries help reveal how galaxies formed? In the series finale, NOVA Wonders journeys to the stars and back to investigate what we know—and don’t know. Find out how scientists are discovering new secrets about the history of the universe, and why they’re predicting a shocking future.
NOVA Wonders: What's The Universe Made Of?
PBS Airdate: May 30, 2018
TALITHIA WILLIAMS (Mathematician, Harvey Mudd College): What do you wonder about?
ERICH JARVIS (Rockefeller University): The unknown.
FLIP TANEDO (University of California, Riverside): What our place in the universe is?
TALITHIA WILLIAMS: Artificial intelligence.
JARED TAGLIALATELA (Kennesaw State University): Look at this. What's this?
KRISTALA JONES PRATHER (Massachusetts Institute of Technology): Animals.
JARED TAGLIALATELA: An egg.
ANDRE FENTON (Neuroscientist, New York University): Your brain.
RANA EL KALIOUBY (Computer Scientist, Affectiva): Life on a faraway planet.
TALITHIA WILLIAMS: NOVA Wonders, investigating the biggest mysteries…
JOHN ASHER JOHNSON (Harvard-Smithsonian Center for Astrophysics): We have no idea what's going on there.
JASON KALIRAI (Space Telescope Science Institute): These planets in the middle, we think are in the habitable zone.
TALITHIA WILLIAMS: …and making incredible discoveries.
CATHERINE HOBAITER (University of St Andrews): Trying to understand their behavior, their life, everything that goes on here.
DAVID COX (Harvard University): Building an artificial intelligence is going to be the crowning achievement of humanity.
TALITHIA WILLIAMS: We are three scientists, exploring the frontiers of human knowledge.
ANDRE FENTON: I'm a neuroscientist, and I study the biology of memory.
RANA EL KALIOUBY: I'm a computer scientist, and I build technology that can read human emotions.
TALITHIA WILLIAMS: And I'm a mathematician, using big data to understand our modern world. And we're tackling the biggest questions…
SCIENTISTS: Dark energy? Dark energy!
TALITHIA WILLIAMS: …of life…
DAVID T. PRIDE (University of Califormia, San Diego): There's all of these microbes, and we just don't know what they are.
TALITHIA WILLIAMS: …and the cosmos.
On this episode…
ALEX FILIPPENKO (University of California, Berkeley, from Runaway Universe): Hey, it's there! We got something!
SCIENTIST 1: The first ever…
TALITHIA WILLIAMS: …the hunt for the secret ingredients of the universe…
FLIP TANEDO: This was a mystery.
SAUL PERLMUTTER (Lawrence Berkeley National Laboratory): We came up with this bizarre result.
DAVID KAISER (Massachusetts Institute of Technology): Most of what astronomers had assumed about our universe fell apart.
TALITHIA WILLIAMS: …the mysterious, invisible forces that control the fate of the cosmos.
ALEX FILIPPENKO: It's 70 percent of the contents of the universe.
MARCELLE SOARES-SANTOS (Dark Energy Survey Collaboration): We have no idea what it is.
DAVID KAISER: Very weird. I mean, it's crazy land!
TALITHIA WILLIAMS: NOVA Wonders: What's the Universe Made Of? Right now!
When you stare up at the sky at night, it's hard not to wonder what's out there in the cosmos.
ANDRE FENTON: Today, we can see incredible things.
RANA EL KALIOUBY: Telescopes gaze at galaxies, far, far away.
TALITHIA WILLIAMS: And we've peered back in time, almost to the beginning of the universe itself.
But in recent years, astronomers made a disturbing discovery: our universe is hiding something.
ANDRE FENTON: Actually, it's hiding a lot.
RANA EL KALIOUBY: It turns out, all the stuff we can see, all that we've come to understand, adds up to only five percent of the universe.
TALITHIA WILLIAMS: The other 95 percent is made up of two mysterious ingredients…
RANA EL KALIOUBY: …dark matter and dark energy.
TALITHIA WILLIAMS: Not only do they make up most of the cosmos, but the two are in an epic battle to control the fate of the universe.
ANDRE FENTON: Today, scientists are on the hunt, trying to understand these dark mysteries…
TALITHIA WILLIAMS: …uncovering new secrets about the history of our universe and predicting a shocking future.
ANDRE FENTON: I'm Andre Fenton.
RANA EL KALIOUBY: I'm Rana el Kaliouby.
TALITHIA WILLIAMS: I'm Talithia Williams. And on this episode, NOVA Wonders: What's the Universe Made Of?
MARCELLE SOARES-SANTOS: I think it was 7:40 in the morning. My phone rings, and my colleague says, "Wake up!"
TALITHIA WILLIAMS: Early on an August morning, at her apartment in Chicago, Marcelle Soares-Santos gets the call she and dozens of other astrophysicists have been waiting for.
MARCELLE SOARES-SANTOS: My colleague says, "We received a signal. We have to take action." And I'm like, "Oh! This, this is really happening."
TALITHIA WILLIAMS: The signal is of vibrations created by a gigantic explosion across the cosmos.
MARCELLE SOARES-SANTOS: We're talking about two neutron stars…
TALITHIA WILLIAMS: One-hundred-and-thirty-million lightyears away, two massive neutron stars have violently crashed together.
MARCELLE SOARES-SANTOS: …very dense objects, colliding at approximately the speed of light. The explosion is gigantic, it's tremendous.
TALITHIA WILLIAMS: Astronomers around the globe rush to their telescopes, hoping to capture the faint light of this distant catastrophe.
On a mountaintop in Chile, some of Marcelle's colleagues point a powerful telescope toward a patch of sky in the constellation Hydra.
MARCELLE SOARES-SANTOS: We expect the light from these sources to fade away quickly, so you have to act fast.
SCIENTIST 2: The data taking has started.
TALITHIA WILLIAMS: As the pictures come in, researchers all over the world sift through the data, looking for one extraordinary dot.
MARCELLE SOARES-SANTOS: The sky is full of beautiful, bright sources, but there will be one that was not there before that is there now.
TALITHIA WILLIAMS: Finally…
SCIENTIST 3: Holy [expletive deleted], look at that!
TALITHIA WILLIAMS: …someone spots something.
SCIENTIST 4: That blob here.
TALITHIA WILLIAMS: Very low on the horizon, there's a light in the sky…
SCIENTIST 4: We found it!
TALITHIA WILLIAMS: …that has never been seen before.
SCIENTIST 4: That really small spot of light…
SCIENTIST 5: That one, right there.
SCIENTIST 4: Very, very cool.
SCIENTIST 5: It's spectacular.
SCIENTIST 5: You don't get many chances like that.
SCIENTIST 2: Yeah.
MARCELLE SOARES-SANTOS: Looking at the screen, and you're like, "This really happening? Is this, is this real?"
TALITHIA WILLIAMS: This tiny blob is the light from that titanic collision in a galaxy far away. Not only is this the first time such an event has been captured, but for Marcelle and her colleagues, this kind of data could help solve a mystery that's perplexed astronomers for years, to decipher the strange, invisible ingredients that make up the vast majority of our universe.
But the clues to solve this mystery aren't just in galaxies deep in space; they could be all around us. In a remote Canadian forest, just north of Lake Huron, another group of scientists is setting a trap. But their snare is not aimed at the sky; it lies in the other direction.
Deep beneath the forest is the Vale nickel and copper mine.
KEN CLARK (SNOLAB): So, we're about 6,800 feet underground.
TALITHIA WILLIAMS: For more than a century, miners here have pulled metal ore out from the surrounding rock, but now another team has come.
KEN CLARK: We're down here, in this mine, because it shields out all the radiation that would make our detectors unusable on the surface.
TALITHIA WILLIAMS: Ken Clark is a different kind of miner, seeking a treasure far more precious.
KEN CLARK: That noise is the ventilation doors. It effectively creates an airlock.
TALITHIA WILLIAMS: These machines are designed to detect a very elusive particle.
KEN CLARK: It doesn't interact with light, we can't see it, but the discovery really could be just around the corner.
TALITHIA WILLIAMS: Ken is one of dozens of scientists here, hunting a substance so mysterious it doesn't even have a name. They call it "dark matter."
KEN CLARK: There's a lot of experiments. We're all kind of racing to try and find this thing.
TALITHIA WILLIAMS: Racing to find it, because scientists believe this mysterious stuff played a key role in shaping the universe as we know it.
Ken and Marcelle are just two in a long line of cosmic detectives, trying to understand how our universe works and what it's made of. It's an investigation that's revealed bigger and bigger surprises, starting about a hundred years ago.
Back then, most scientists, even Albert Einstein, thought the entire universe consisted only of this: a single galaxy, the Milky Way, sitting in space. But this small, simple universe was about to be blown to smithereens.
The telescopes back then were small, but besides stars, they could make out faint glowing clouds, which scientists believed were made of gas and dust. They called them "nebulae."
One scientist, Edwin Hubble, decided to take a closer look.
PRIYAMVADA NATARAJAN (Yale University): What Hubble needed to do was to actually measure the distance to these nebulae.
TALITHIA WILLIAMS: Using one of the most powerful telescopes of the day, Hubble was able to pick out stars in these nebulae and calculate their distances. To his amazement, he found that they were over four-times farther away from us than any star seen before in our Milky Way.
DAVID KAISER: The distances from us were truly astronomical. These nebulae, they weren't just smears of gas, they were, indeed, collections of stars all their own, outside of our galaxy.
TALITHIA WILLIAMS: Hubble realized that this nebula, known as Andromeda, wasn't a cloud of gas at all; it was another galaxy. And it wasn't the only one.
DAVID KAISER: We learned that the Milky Way galaxy is one of a vast sea of galaxies, hundreds of billions, maybe more.
SAUL PERLMUTTER: Galaxy, after galaxy, after galaxy, in every direction, down, up, sideways, in an infinite universe.
TALITHIA WILLIAMS: What's more, Hubble, along with other astronomers, could see that these galaxies were on the move, rapidly flying away from us.
ALEX FILIPPENKO: Hubble found that the universe isn't static after all. It's expanding.
DAVID KAISER: Most of what astronomers had assumed about our universe fell apart.
KATHERINE FREESE (University of Michigan): It was a complete paradigm shift. It was a complete shock to everybody.
PRIYA NATARAJAN: …pretty disorienting.
TALITHIA WILLIAMS: Far from being confined to a single galaxy, the Milky Way, the universe was filled with galaxies, and they were all on the move.
RANA EL KALIOUBY: Hubble's discovery means that the universe is big and getting bigger all the time, with galaxies flying away from each other. But if that's true, if everything in the universe is flying apart right now, what did it do in the past? What would happen if you ran the clock backwards?
FLIP TANEDO: Essentially, what we're doing is we're playing the tape backwards and we're saying, if the universe is getting bigger now, it must have been small earlier. It must have been really, really small a long time ago.
TALITHIA WILLIAMS: Keep rewinding, and everything gets closer and closer together.
SAUL PERLMUTTER: Eventually, they get so dense that you have a soup of elementary particles.
DAVID KAISER: All the stuff we see around us was compacted to literally a single point. All of space was a little tiny dot.
TALITHIA WILLIAMS: This is how it all started…
ALEX FILIPPENKO: We don't necessarily know why it started that way, but it started out as this very, very small region of high density.
TALITHIA WILLIAMS: The Big Bang: as the clock ticks forward now, in the very first fraction of a fraction of a second, scientists think the universe went through an intense period of expansion.
DAVID KAISER: We call that era "cosmic inflation."
BRIAN NORD (Fermilab): The initial stages are like a growth spurt. So, there's this period of inflation, where the universe's size grew really, really fast.
DAVID KAISER: …ripping apart at an enormous, enormous exponential rate.
TALITHIA WILLIAMS: As it cools, the growing universe condenses into a soup of exotic particles.
FLIP TANEDO: The universe is a hot, dense plasma, it's a hot gas; there's particles, there's anti-particles; they're coming in and out of existence.
TALITHIA WILLIAMS: The seconds tick by. The soup of particles remains unsettled. Three-hundred-eighty-thousand years pass by. As the universe keeps expanding, it cools.
MELISSA FRANKLIN (Harvard University): Then you get atoms, because things are cooling enough that atoms can actually form where you have protons and neutrons in the center, and electrons around them.
DAVID KAISER: For the first time in cosmic history, the temperature falls just low enough, and that changes things forever.
TALITHIA WILLIAMS: Finally, light can travel across space. This is a snapshot of that moment, a baby picture of the universe.
ALEX FILLIPENKO: An image of the universe when it was just an infant, 380,000 years after the Big Bang. Now, that may sound like a long time on a human timescale, but compared with the age of the universe, 13.8-billion years, it's just an instant, near the very beginning.
TALITHIA WILLIAMS: Already, the blue areas reveal where matter will clump together, forming the seeds that will grow into galaxies. The first stars are born and die and re-form, in a cycle generating the building blocks for planets, ever more complex chemistry, and eventually us.
But why did galaxies begin to form at all? The energy that was expanding the universe ever since the Big Bang should have spread the little bits of matter too thin.
DAVID KAISER: So, as the universe continues to expand, we might have expected these little tiny lumps in the universe to really get smoothed out. Instead, we know the opposite happened.
TALITHIA WILLIAMS: How could so much of the matter clump together to form the major structures of the universe? It's a question that's plagued astronomers for decades. The first clue came from a Swiss astronomer named Fritz Zwicky, just a few years after the discoveries that had suggested the Big Bang.
Zwicky noticed that these newly discovered galaxies were behaving oddly.
ALEX FILLIPENKO: Fritz Zwicky looked at clusters of galaxies and found that the individual galaxies within those clusters are moving so fast, that the clusters should fly apart.
PRIYA NATARAJAN: Moving around so rapidly that it was impossible to understand why they didn't just wander away. Something clearly held them in these orbits.
TALITHIA WILLIAMS: Zwicky could see nothing in his telescope to explain it, so he called the phenomenon "dunkle materie,"translated as "dark matter." And then the idea promptly faded away.
Zwicky's observation might have ended up forgotten. And for nearly forty years, it was, until an astronomer named Vera Rubin entered the field.
FLIP TANEDO: Vera Rubin was one of these astronomers who was not appreciated until much later. She was a woman in astronomy at a time when the field was not particularly friendly to women.
TALITHIA WILLIAMS: Rubin chose to work in a relatively quiet area of astronomy: making straightforward measurements of stars as they orbited in their galaxies.
VERA RUBIN: Here's what we get.
TALITHIA WILLIAMS: But she, too, noticed something bizarre happening.
VERA RUBIN: The stars way out here are going very fast.
TALITHIA WILLIAMS: The stars at the edge of the galaxies were moving so fast, that they should have been flung off into space.
FLIP TANEDO: This was a mystery: that these stars were moving too fast to be explained by ordinary matter.
ANDRE FENTON: Think about a spinning wheel, covered in water. If the wheel is moving slowly, the water clings to the wheel, but spin it fast enough, the water flies off. The same thing should happen out in the universe. Stars swirling around in a galaxy, if they orbit too fast, they'll get flung off, out into space.
Except that's not what Vera Rubin sees.
TALITHIA WILLIAMS: The galaxies are spinning fast, but the stars stay in their orbits. What's holding them there? It has to be gravity.
VERA RUBIN: A response to a gravitational pull from something that's not bright. And we don't know what that is.
TALITHIA WILLIAMS: But gravity doesn't exist alone, it depends on stuff: matter and energy. Vera Rubin knew that gravity is produced by mass. Einstein had proven it.
DAVID KAISER: The main takeaway message of Einstein's general theory of relativity is that gravity is nothing but the warping of space and time. Space-time itself becomes something like a fabric that, when we put objects like galaxies within this fabric of space-time, it will warp.
TALITHIA WILLIAMS: Massive objects create hills and valleys in the fabric of space, and these create gravity.
FLIP TANEDO: The one thing that we know is that if you have stuff with mass, stuff with energy, it's going to pull planets, it's going to pull stars, it's going to pull other galaxies.
TALITHIA WILLIAMS: The amount of gravity all depends on the amount of mass. The more stuff rolling around in the fabric of space, the more distortion, the more gravity. It was clear to Vera Rubin that a lot of gravity was holding the stars in place, but there wasn't enough stuff, enough visible matter to generate so much gravity. There must be some missing matter.
Dark matter was real.
KATHERINE FREESE: It doesn't shine. It doesn't give off light.
FLIP TANEDO: By definition it is the stuff that we have a really hard time being able to quantify. That's why they called it dark matter.
TALITHIA WILLIAMS: The more astronomers looked, the more dark matter there seemed to be. But how much is there? And where exactly is all this mysterious stuff?
Astrophysicist Priya Natarajan is trying to find out.
PRIYA NATARAJAN: I have worked my entire career on trying to understand the nature of dark matter.
TALITHIA WILLIAMS: But how do you understand what you can't see? Luckily, this invisible dark matter gives itself away, because it has a habit of playing tricks with light.
PRIYA NATARAJAN: In 2014, with the Hubble Space Telescope, a very intriguing kind of object was observed.
TALITHIA WILLIAMS: It appeared to be a galaxy with four exploding stars, called supernovae, going off at the same time.
PRIYA NATARAJAN: Like four evenly-spaced supernovae.
TALITHIA WILLIAMS: In reality, there's only one supernova, but it somehow shows up in four different places. What's going on?
PRIYA NATARAJAN: This configuration of four evenly-spaced multiple images is called an "Einstein cross." It was predicted by Einstein. In reality, one supernova went, "whoop," and, we had a little gift. The paths of light rays are bent into a configuration with four distinct images of the same supernova.
TALITHIA WILLIAMS: Somehow the light from that one supernova traveled along several bending pathways, arriving at four different spots in the sky.
PRIYA NATARAJAN: The phenomenon of light bending is something we actually encounter every day, and it's all around us. So, for example, if you look at, say, graph paper through the bottom of a wine glass, you know this is a regularly spaced grid, but because of the light bending, you can actually see a stretching of the grid pattern.
TALITHIA WILLIAMS: In the cosmos, what bends light is gravity, distorting the fabric of space. It's called gravitational lensing, and it can produce spectacular results: smears, rings, smiley faces. It can even make a supernova show up in four different places at once.
For Priya, these aren't just fascinating illusions, they are crucial clues in the dark matter mystery. Since gravity is what bends the light in these images, and dark matter creates gravity, the distortions can reveal where dark matter is in the universe.
PRIYA NATARAJAN: And so, it's the dark matter that is producing this huge amount of distortion.
TALITHIA WILLIAMS: So, Priya is gathering a giant database of these distortions, all in her quest to map out dark matter throughout the universe. And Priya and maps? Well, they go a long way back.
PRIYA NATARAJAN: We're going to one of my favorite places, where I fulfill all my childhood fantasies, the map room at the Beinecke Rare Book Library.
TALITHIA WILLIAMS: Priya's quest grew from an obsession that's gripped her since she was a young girl.
PRIYA NATARAJAN: I was obsessed with all kinds of maps and atlases when I was young. I'm, like, I'm crazy about maps.
These mappers of yore, when they ran out of data or knowledge, it was marked as "terra incognita," mythical places that await exploration.
TALITHIA WILLIAMS: The places that young Priya most wanted to map were not on Earth, but in the heavens.
PRIYA NATARAJAN: There was something about the cosmos being a little bit out of reach that really attracted me.
TALITHIA WILLIAMS: As soon as she got her first computer, she used it to create a star chart.
PRIYA NATARAJAN: It was a hard problem, and I sat down for six weeks; I wrote the program. These were not things that no one had figured out before, right? But I was figuring them out for the first time. I was hooked.
TALITHIA WILLIAMS: Today, Priya is fulfilling her dream of exploring the frontiers of the universe. She's one of several researchers writing computer programs that use gravitational lensing to map the location of dark matter.
PRIYA NATARAJAN: This is one of the largest maps of dark matter. The red regions are where you have an excess of dark matter. If we zoom in to a dark matter simulation, it looks rather like these fibers, almost like neurons.
TALITHIA WILLIAMS: Using computer simulations of the early universe, astronomers now think that dark matter formed a giant web.
PRIYA NATARAJAN: Where the dark matter filaments cross, at these nodes, you form these clusters of galaxies.
TALITHIA WILLIAMS: Astrophysicists now realize dark matter must have played an essential role early on, drawing together ordinary matter and allowing galaxies to form.
We wouldn't be here, if it weren't for the powerful pull of dark matter.
PRIYA NATARAJAN: Our current understanding of dark matter is literally it shapes the universe that we see.
TALITHIA WILLIAMS: And what's clear: there's a ton of it.
DAVID KAISER: By now, we actually have many independent measures, many independent ways to estimate the total amount of dark matter in the universe. And amazingly, each of them points to an amount of something like five- or five- to six-times more dark matter than ordinary matter. For every atom of ordinary matter, there seems to be five times more mass in some mysterious dark matter throughout the entire universe.
TALITHIA WILLIAMS: Let's say I'm made of ordinary matter, the stuff we see and understand, like atoms. Now, add dark matter, and it's as if, for every one of me, the universe has about five more, made of entirely different stuff.
They're there but completely invisible. We only know they exist because of their gravity. It seems totally bizarre and kind of freaky, yet that's what the universe is telling us. The vast majority of matter is this mysterious stuff, dark matter.
But what is it?
FLIP TANEDO: I can't imagine that dark matter is fire-breathing dragons that spit…that come out of black holes to eat us. It's definitely not that. But could it be a heavy particle? Could it be a light particle? Can it do exotic things? Maybe it's something really boring. I don't know.
TALITHIA WILLIAMS: These kinds of questions are nothing new. People have been wondering about what exactly matter is for millennia. But only recently have we had the tools to actually figure it out.
JOSEPH LYKKEN (Fermilab): A hundred years ago, in a sense, all matter was dark matter, because we didn't have the technology to pull apart what these particles are that everything is made out of.
TALITHIA WILLIAMS: In the early 20th century, while Hubble was peering up at the cosmos, other scientists were focused on the tiny world of atoms, trying to decipher the nature of matter itself. They devised enormous machines, called "accelerators," to break atoms into their constituent parts. Accelerators revealed a zoo of elementary particles with all sorts of whimsical names.
JOSEPH LYKKEN: Particle names, some of them are cute, like "neutrino," which I think is, is one of the best names.
SAUL PERLMUTTER: "Quarks."
JASON ST. JOHN (Fermilab): You have "up" and "down quarks."
JOSEPH LYKKEN: "Top quark."
DAVID KAISER: "Bottom quark."
JASON ST. JOHN: "Charmed" and "strange quarks," and "truth" and…
PRIYA NATARAJAN: …"beauty quark."
BRIAN NORD: "Gluino."
MELISSA FRANKLIN: "Electron."
BRIAN NORD: "Photino."
MELISSA FRANKLIN: "Photons."
SAUL PERLMUTTER: "Gluons."
PRIYA NATARAJAN: "Pion."
JASON ST. JOHN: "Kaons."
DAVID KAISER: "Upsilons."
JOSEPH LYKKEN: The "Higgs boson."
BRIAN NORD: Oh, "positron," actually. "Positron" is great.
TALITHIA WILLIAMS: Through decades of experiments, physicists have figured out so many recipes describing what the universe is made of at the tiniest of scales. Groups of quarks make a proton; protons, neutrons and electrons make atoms; atoms combine to make molecules. Together, they make the stuff we know and love.
Today, the biggest particle accelerator is at CERN, near Geneva, Switzerland, where physicists recently detected a new particle, the Higgs boson, which gives normal matter its mass. And they're still looking for more. The question is, is dark matter anything like ordinary matter?
DAVID KAISER: Is dark matter some other kind of particle we just haven't detected, haven't found yet?
TALITHIA WILLIAMS: The answer must lie at the intersection of particle physics and astronomy. Peter Fisher was one of the first to bring particle physics to the dark matter problem.
PETER FISHER (Massachusetts Institute of Technology): Finding out what dark matter is, has been something that's really driven more by particle physics than by astronomers.
TALITHIA WILLIAMS: For decades, physicists like Peter have focused on a theoretical particle called a "WIMP."
PETER FISHER: Weakly Interacting Massive Particle, or WIMP. I think there was a lot of work that went into finding that acronym.
TALITHIA WILLIAMS: In order to create the kind of gravity that draws large amounts of matter together, the particle would need to have mass, but because it's invisible and eludes detection, it also must be "weakly interacting."
PETER FISHER: So, I think of dark matter as, kind of, ghosts. We don't see them, because they just don't interact very often. What that means, is that a, a WIMP could pass right through the earth, without hitting any of the atoms in the earth. In fact, if you lined up a hundred-billion earths, a WIMP would go right through.
TALITHIA WILLIAMS: So, how do you capture such an elusive particle? Peter Fisher spent 20 years building machine after machine, attempting to do just that.
PETER FISHER: My students, postdocs and I have built hundreds of these different experiments, hundreds.
There's the remnants of three sitting right here. This is really kind of a mess. Every experiment we build is bigger and more complicated.
TALITHIA WILLIAMS: And with each generation, the experiments not only got larger and more complex, they went further underground.
The hunt for WIMPs brought particle physicist Ken Clark here, to this mine in Canada.
KEN CLARK: We try to detect them in a much more physical way. We're actually looking for this dark matter to interact. And that's what most of the major dark matter experiments right now are trying to do.
TALITHIA WILLIAMS: There are four different experiments at SNOLAB. At 6,800 feet underground, it is one of the deepest labs in the world. It has to be.
KEN CLARK: All the time, all around us there's cosmic rays and there's particles that are streaming in through the earth's atmosphere, and that kind of thing. And if we were to set up our experiments here, on the surface, we would be completely swamped by those signals.
TALITHIA WILLIAMS: Instead, the experiments are brought here, a mile underground, into special caverns blasted out of the bedrock. The laboratory functions as one giant cleanroom, to keep the experiments free from interference.
KEN CLARK: One fingerprint on the experiment would make it unusable. It would be too dirty for us to actually use.
TALITHIA WILLIAMS: The largest of the caverns down here houses the DEAP 3600 experiment.
STEFANIE LANGROCK (SNOLAB, Laurentian University): It's the biggest liquid argon dark matter detector currently in operation.
SNOLAB STUDENT: So, this is our cryocoolers, right now. They keep the temperature at negative-200 degrees Celsius in the detector.
TALITHIA WILLIAMS: Inside this huge vat is the liquefied gas, argon. It has to be kept extremely cold, almost at absolute zero. Inside, the idea is that the argon atoms are so cold, they are barely moving. If any foreign particle were to fly through the argon, even if it were weakly interacting, it might hit one of the argon atoms, setting off a chain reaction and trigger a detection. So far, the huge ultracold experiments have yet to yield any dark matter.
PIERRE GOREL (SNOLAB): The dark matter from outer space, so far, has been missing.
STEFANIE LANGROCK: None.
TALITHIA WILLIAMS: Just down the hall, Ken Clark's experiment takes a slightly different approach. He's not freezing things, he's looking for them to boil.
The experiment starts with a container full of superheated liquid made of carbon and fluorine. It's placed under high pressure to keep it from boiling.
KEN CLARK: Which means it's at a temperature above its normal boiling point at this pressure, so any little deposit of energy means it boils instantly.
TALITHIA WILLIAMS: Under these conditions, if a particle enters the liquid from outside, it could immediately push the liquid past the boiling point.
KEN CLARK: We're looking for the dark matter particle to come in, hit one of the fluorine nuclei, cause it to recoil that tiny bit, and then cause a bubbling here.
TALITHIA WILLIAMS: Custom-designed cameras are constantly filming, waiting for a bubble. But they haven't found a WIMP yet.
KEN CLARK: So far, this one has detected exactly zero dark matter particles. But we're hopeful that the next generation, we're going to actually see something in it.
TALITHIA WILLIAMS: Back at M.I.T., it's a familiar story for Peter Fisher.
PETER FISHER: In hundreds of experiments, we've, we've never seen what we know to be a WIMP.
I've been doing this for 35 years. And, and so you might think that you know not having detected a WIMP, I would be frustrated by that. Maybe a little bit, but as a scientist, what's exciting is, is building something and seeing it work. Someday, these ideas might really shape how we think of ourselves as, as, as living beings.
TALITHIA WILLIAMS: We may not yet know what dark matter consists of, but we do know what it's been doing.
Ever since the Big Bang, dark matter's gravity has been drawing the universe together. Once astronomers realized this, they began to wonder what this might mean for the future.
DAVID KAISER: We know the universe is filled with ordinary matter; it's chock full of dark matter; the gravitational tug of all that matter should have, sort of, slowed the rate at which the universe as a whole continued to expand. Maybe the expansion itself could literally halt, maybe even leading to a reverse Big Bang, a "Big Crunch."
RANA EL KALIOUBY: Think about the simple act of throwing a ball. Every time I throw the ball up, gravity will slow it down and, at some point, will pull it back to Earth. So, could this happen to all the stuff in the universe?
We know that everything in the universe is flying outward right now, but how long will that last? Could it be like this ball? Slowing down, eventually reversing direction and returning to where it came from? What would that mean for the future of the universe?
TALITHIA WILLIAMS: In the late 1990s, two groups of astronomers, including Saul Perlmutter and Alex Filippenko…
ALEX FILIPPENKO: Eight-point-eight-three arc seconds north.
TALITHIA WILLIAMS: …were trying to answer that very question. As the universe was expanding, would gravity slow it down and eventually pull it back together?
ALEX FILIPPENKO: The original goal of our project was to measure the rate at which the expansion of the universe is slowing down.
TALITHIA WILLIAMS: They set out to measure the speed of the universe as it expands outward and detect how much it's slowing down. But how do you do that?
Turns out, there's a kind of star that's perfect for this measurement: a supernova.
WHO'S THIS: Right there?
ALEX FILIPPENKO: Oh, yeah, it might be right there.
A supernova is simply an exploding star. Now, most stars, like our sun, will die a relatively quiet death, but a small minority literally destroy themselves in a titanic explosion at the end of their lives, becoming millions or even several-billion-times as powerful as our sun.
SAUL PERLMUTTER: Because they're so bright, this one object can be seen 10-billion lightyears away and, and further, so, already that's interesting.
TALITHIA WILLIAMS: The team needed a particular kind of supernova, called a type 1a. Their explosions always reach a certain peak brightness, allowing astronomers to calculate their distance from Earth. Like headlights on a road, the dimmer they appear, the farther away they must be. But first astronomers had to find them.
ALEX FILIPPENKO: Is it there, or is it not?
Supernovas are pretty rare: roughly once per galaxy, per century, or even per several centuries.
TALITHIA WILLIAMS: Astronomers had to survey thousands of galaxies at once, looking for this needle in a cosmic haystack.
ALEX FILIPPENKO: In five minutes, we'll know.
TALITHIA WILLIAMS: Months of grueling observation eventually yielded a handful of 1a supernovae…
ALEX FILIPPENKO: Hey, it's there! We got something!
TALITHIA WILLIAMS: …from various times in the history of the universe.
ALEX FILIPPENKO: Okay, let's keep on exposing.
TALITHIA WILLIAMS: Not only could they determine their distance, but the teams could also gauge how fast they were travelling as the universe expanded.
They did this by measuring something called "redshift." Redshift is caused when light travels across regions of space that are expanding. As the fabric of space stretches, so, too, does the wavelength of light, shifting toward the red end of the spectrum.
By analyzing the redshift of different supernovae, the teams could see how fast the universe was expanding and stretching at different times in its history.
SAUL PERLMUTTER: We ended up with a pool of some 42 supernova, mapping out the history for some seven-, eight-billion years, to see when was it expanding faster, when was it expanding slower? And we looked to see whether it was slowing down enough to come to a halt.
ALEX FILIPPENKO: Okay, here we go.
TALITHIA WILLIAMS: But when they finished processing their data…
BRIAN SCHMIDT: I'd be a little suspicious of that one, guys.
TALITHIA WILLIAMS: …something did not look right.
SAUL PERLMUTTER: We actually finally made the measurement, we came up with this bizarre result.
TALITHIA WILLIAMS: The supernovae were much farther away than they expected, meaning the stars and their galaxies were travelling much faster than anyone predicted.
DAVID KAISER: As they pieced these pieces of the puzzle together, the teams found, much to their own surprise, to the real tremendous surprise to the community at large, was that the universe is not slowing down in its expansion at all. Instead, these surveys showed that the universe is speeding up in its rate of expansion. It's not just still expanding, it's expanding faster and faster over time.
TALITHIA WILLIAMS: The universe was not just expanding, it was accelerating.
ALEX FILIPPENKO: And that's like, oh, my gosh, in a multiple choice test, that's not one of the options. And my jaw just dropped.
KATHERINE FREESE: This is revolutionary. The universe is accelerating?
MELISSA FRANKLIN: A ball, if I had a ball, and it just started moving in that direction, moving faster and faster and faster, but no one was throwing it or there was no force that would be weird, right? That would be weird.
TALITHIA WILLIAMS: Some unknown force was pushing the universe apart, challenging everything we thought we knew about the cosmos. Scientists dubbed it "dark energy." And they soon determined there was a lot of it.
ALEX FILIPPENKO: It's 70 percent of the contents of the universe, 70 percent.
DAVID KAISER: Seven zero; it's far and away the largest contributing factor to all the stuff we can otherwise add up in the universe today.
TALITHIA WILLIAMS: So, what exactly is this weird stuff that makes up the vast majority of our universe?
ALEX FILIPPENKO: In a sense, dark energy is a term that illustrates our ignorance of what's actually out there. We don't know what it is.
MELISSA FRANKLIN: This is a case where it's kind of a mystery. And, and even hard to think about, even for normal physicists.
TALITHIA WILLIAMS: One idea is that the energy comes from some undiscovered particle; another says our understanding of gravity is not quite right. And then there's the most popular theory.
ALEX FILIPPENKO: Perhaps the simplest, and one which is not yet ruled out, is that the dark energy is simply the energy associated with the vacuum of space. It's just part of space itself, it's not something in space, it's just part of what space is.
TALITHIA WILLIAMS: But it's a part of space that creates more space, over and over again.
DAVID KAISER: It almost sort of feeds on itself. So, dark energy is what's stretching the universe at a faster and faster rate, and it is literally making more space, and dark energy is an energy of empty space. So, it's made more empty space, which has in its own more dark energy.
MARCELLE SOARES-SANTOS: It's the only form of energy that we know that is capable of doing that. To make space-time expand faster, and faster, it's very weird.
DAVID KAISER: It's crazy land! It's, it's very weird!
MARCELLE SOARES-SANTOS: We have no idea what is the physics underlying it.
TALITHIA WILLIAMS: Marcelle Soares-Santos is trying to figure out the physics behind dark energy.
MARCELLE SOARES-SANTOS: Because it's really about figuring out something that we have no idea what it is.
TALITHIA WILLIAMS: She's part of the Dark Energy Survey, an international research initiative.
JOSH FRIEMAN (Dark Energy Survey Collaboration): We want to know, really, what is the precise nature of dark energy?
Okay, its redshift is point-two, so…
TALITHIA WILLIAMS: Josh Frieman leads one of the teams, based at Fermilab, outside Chicago. The strategy is to try to track how fast the universe is accelerating, as precisely as possible.
JOSH FRIEMAN: Turns out that the more precisely we can measure how fast the universe is expanding today, the better job we'll do in trying to figure out what dark energy really is.
TALITHIA WILLIAMS: Back in the 1990s, the discovery of dark energy was based on just a few dozen supernovae, but today, the dark energy survey can do much more. Powerful telescopes, like this one on a mountaintop in Chile, scan huge swaths of the sky. With so many images and powerful computers to analyze them, the team has collected thousands of new supernovae, each one a snapshot of a different point in the universe's history.
But there's another set of clues that might help paint a clearer picture of this mysterious dark energy, and that's why Marcelle was so excited when signs of a gigantic cosmic explosion recently reached Earth.
MARCELLE SOARES-SANTOS: This was something that we were all preparing for a long time.
TALITHIA WILLIAMS: A-hundred-and-thirty-million lightyears away, two neutron stars had collided. The explosion was so powerful, it sent gravitational waves, ripples in the fabric of space-time, across the universe.
MARCELLE SOARES-SANTOS: We received a signal from LIGO and Virgo.
TALITHIA WILLIAMS: Like the ringing of a bell, the waves trigger sensors here on Earth, at LIGO in the U.S. and Virgo in Italy. Astronomers around the world point their telescopes towards the source of the signal, trying to find the light from the explosion.
MARCELLE SOARES-SANTOS: We're looking for the light corresponding to that sound that the gravitational wave detectors just heard.
TALITHIA WILLIAMS: And then, they find it: from Earth, a tiny dot that wasn't there before. Several research teams around the globe spot this dot.
MARCELLE SOARES-SANTOS: Oh, this was fantastic.
TALITHIA WILLIAMS: Fantastic because, for the first time, astronomers both hear and see a distant cosmic event.
SCIENTIST 1: That is the first ever, for any astronomer.
TALITHIA WILLIAMS: That alone is remarkable, but for the Dark Energy Survey, this type of event has opened a new window on the universe.
MARCELLE SOARES-SANTOS: With the gravitational wave data, we can do more. The gravitational wave's signal contains information about the distance to the source that is independent from the light.
TALITHIA WILLIAMS: Gravitational waves provide a whole new source of information, helping to pinpoint the distance to these violent collisions. And having two independent sources, both seeing and hearing an event could reveal how fast the universe was stretching apart at the moment of the explosion.
MARCELLE SOARES-SANTOS: Now we have a new way to attack the problem. We can determine how fast the universe is expanding in between, and voila, we have information about dark energy.
TALITHIA WILLIAMS: For the team, combining gravitational signals with more tiny dots like this one might someday help reveal what dark energy actually is.
MARCELLE SOARES-SANTOS: Everybody knew that we were, in some sense, witnessing the birth of a new field and a new area of research.
TALITHIA WILLIAMS: Astronomers may not have cracked the dark energy mystery, but the last 20 years have uncovered a new dramatic story: the ongoing epic struggle across the cosmos between dark energy and dark matter.
Astronomers are convinced that these are the two major players in the universe: dark matter, pulling the universe together, and dark energy, pushing the universe apart. They're engaged in a cosmic tug of war that will determine nothing less than the fate of our universe.
DAVID KAISER: They're really, sort of, literally pulling in opposite directions. So, we know that dark matter and dark energy are, are in the grips of this cosmic competition, and which side, so to speak, has been winning has itself changed over time.
TALITHIA WILLIAMS: With each discovery, we're getting a clearer picture of how this battle has played out since the birth of the universe. Just after the Big Bang, the universe was literally a hot mess, sizzling with radiation until dark matter and matter formed. Dark matter and its gravity became the dominant driver in the universe, pulling together gas and dust, allowing galaxies and stars to form.
PETER FISHER: And there was a time where normal and dark matter dominated the universe.
TALITHIA WILLIAMS: In fact, for nearly nine-billion years, dark matter's gravity was so strong, it was slowing down the expansion of the universe. But then something changed. About five-billion years ago, the universe started accelerating in its expansion. This moment, when the universe stopped slowing down and suddenly started speeding up, is known as the "cosmic jerk."
PETER FISHER: And really, starting just a few billion years ago, dark energy came to dominate the universe, so I would say we have evolved into a dark universe.
TALITHIA WILLIAMS: Around the world, researchers continue to hunt, determined to find the secret ingredients that make the universe and everything in it possible.
MELISSA FRANKLIN: You have people looking on all sides. And somehow, all of those things together are going to help us to understand. It's kind of an incredibly great example of how science should really work, that everybody should just follow their own curiosity and intuition, and then, together, it'll be brilliant.
MARCELLE SOARES-SANTOS: It is a little bit humbling to look out there in the universe and say, "Most of it, I don't understand." But, at the same time, of the part we do understand, we understand it so well that we were able to transform the world around us based on our knowledge. And all of that success makes us confident that we will succeed here, as well.
JOSEPH LYKKEN: We're just scratching the surface. The whole history of science is finding out the universe is bigger and more complicated and more mysterious than anybody had thought. We found out the earth was a planet, and then we had a solar system, then we have a galaxy, then we have billions and billions of galaxies. Where's the end of that? We don't know. That is a big mystery.
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- Ken Clark, Rana el Kaliouby, André Fenton, Alex Filippenko, Peter Fisher, Melissa Franklin, Katherine Freese, Josh Frieman, Pierre Gorel, David Kaiser, Stefanie Langrock, Joseph Lykken, Priyamvada Natarajan, Brian Nord, Saul Perlmutter, Marcelle Soares-Santos, Jason St. John, Flip Tanedo, Talithia Williams