Only about 5% of the universe is made up of stuff that we're familiar with.

Protons, neutrons, electrons, quarks, and all the other matter that make up our bodies, the planets, the stars, everything we've ever seen, is only a tiny fraction of what makes up our entire universe.

85% of the universe's mass is made up of something called dark matter.

And we don't even know what it is.

PADDY FOX: So why is it called dark matter?

So it's called matter because it gravitates in the same way you and I do.

And it's called dark because we've not seen it very well.

CRAIG BENZINE: This is Paddy Fox, theoretical physicist at Fermilab, which is home to the world's second-largest particle accelerator, and where lots of cool science stuff happens.

PADDY FOX: Basically, it doesn't reflect light and it doesn't emit light.

But it also doesn't interact in any other way, as far as we can tell, very, very strongly.

So it interacts very, very, very, very weakly.

That basically means it can fly through stuff with impunity.

And that's why it's so hard to measure.

But we know it's out there.

How do you know that?

PADDY FOX: That's a good question.

So we know that from the way it interacts gravitationally.

So basically, all matter pulls stuff towards it.

You and I are being pulled towards each other right now, very, very weakly by gravity.

And although we don't know what makes up dark matter, we know how it interacts in terms of gravity.

And we've seen its effects gravitationally on the cosmos at large.

Invisible matter was first postulated in 1932 by Jan Oort.

But it wasn't until a year later that Fritz Zwicky, while measuring the gravitational mass of the Coma Cluster of galaxies, noticed something odd.

By measuring the velocity of the orbiting galaxies, Zwicky was able to calculate the mass of the cluster.

However, the mass he calculated was far greater than the observable stars in the galaxies.

His results seemed to suggest that there's more matter out there than could be seen.

He called this Dunkle Materie or dun-clay materie-ay or something, which translates to dark matter.

However, for the time being, his results were largely ignored.

CRAIG BENZINE: In the '60s and '70s scientist Vera Rubin noticed a difference between the angular motion of the galaxies she predicted and the motion she observed.

This was a huge problem.

Basically, Rubin found the galaxies were spinning too fast-- so fast, in fact, they should be flinging all their stars out into outer space.

But they weren't, obviously.

MATT WEBER: This came to be known as the Galaxy Rotation Problem.

To solve this problem, Rubin calculated that there had to be a bunch of matter that they couldn't see keeping the stars gravitationally bound to their galaxies-- as much as 10 times as much matter.

CRAIG BENZINE: While at first met with skepticism, her work was eventually verified.

And it became the strongest evidence we had for the existence of dark matter.

And since then, we've seen more and more evidence of dark matter.

Although we've never directly seen dark matter itself, we can infer its presence by how it affects regular matter.

CRAIG BENZINE: And its effects are pretty noticeable.

We can see it in the way it affects the microwave background radiation left over from the big bang, and how it has shaped the large-scale structure of the universe itself.

Galaxies wouldn't even exist if it weren't for the extra pull of dark matter.

So PADDY FOX: We've inferred the existence of dark matter indirectly from those effects.

But we've never really got our hands on the stuff in the lab in a very direct way yet.

If we believe this dark matter stuff is basically everywhere-- it pervades the galaxy we live in, that in fact our galaxy sits inside a cloud of this dark matter stuff.

And so we'd like to see if they are particles, whether they come in and interact with anything in the lab.

The problem, though, with detecting dark matter is that it's pretty much undetectable.

The hypothetical particles that make up dark matter are called WIMPs.

WIMPs stands for Weakly Interacting Massive Particles.

And since they only weakly interact with ordinary matter, we have a hard time knowing that they are there.

They go straight through anything, including our detectors.

MATT WEBER: Regular matter, on the other hand-- the matter we're made up of-- does a fantastic job of interacting with itself.

Isn't that right, Craig?

CRAIG BENZINE: Ho, ho, hey, ho, ha.

Yeah, just having a little fun with-- CRAIG BENZINE: Interacting matter.

PADDY FOX: I mean, I can't put my hand through this wall, because I interact too strongly.

CRAIG BENZINE: It's good to know that you're not dark matter.

PADDY FOX: Yeah, right.

Exactly.

But since ordinary matter interacts with itself so much, it can act as noise, triggering sensitive detectors, mucking up results, and basically drowning out any dark matter particles that might be coming through.

So you have to stop all the ordinary stuff from interacting with your experiment.

And that includes high-energy particles like cosmic rays that are raining down on Earth all the time.

And the best way to do this is the put as much earth between your experiment and all the cosmic rays raining down on the planet.

In other words, put your experiment as far underground as possible.

Little shaky.

I'm not claustrophobic or anything, though, so I'm OK. You guys, anybody here claustrophobic?

Uh-oh.

How do you do, man?

ROBERTO VEGA-MORALES: I try to pretend I don't know where I am.

CRAIG BENZINE: So what is this?

BILL LEE: So this is the DM-Ice experiment.

This is a dark matter experiment.

What they've done is they put a sodium iodide crystal inside this lead coffin.

CRAIG BENZINE: And it's poisonous.

BILL LEE: Lead is poisonous.

CRAIG BENZINE: That's true.

It's a good point.

It's like they just had a little extra room under the stairway and they-- oh, let's experiment with dark matter.

BILL LEE: We put experiments wherever we can.

The real experiment is at the South Pole, where they have put it in with the IceCube experiment, about two miles under the ice there.

The Ice Cube he's talking about is not the star of "XXX, the State of the Union" or "Are We There Yet?"

It's the IceCube Neutrino Observatory, which is a gigantic telescope designed to detect neutrinos which are made of ordinary matter, but like dark matter, are notoriously hard to capture.

And instead of being buried underground, the IceCube's neutrino detector is buried under two kilometers of ice, just like Ice Cube.

The IceCube hopes to get indirect evidence of dark matter through the detection of excess neutrinos.

Theoretically, since dark matter particles have mass, they should be gravitationally attracted to the sun.

Enough dark-matter particles should be trapped by the sun and collect in its core to annihilate with each other, giving off neutrinos.

The IceCube should be able to infer this dark matter's existence by detecting occasional unexplained neutrino collisions and tracing them back to the source.

There are also experiments measuring the release of gamma rays at the center of the galaxy, and looking for any excess that might suggest the presence of dark matter.

But they've never directly detected a dark matter particle.

The Cryogenic Dark Matter Search, which is located deep in a mine in northern Minnesota, has picked up several interesting signals since 2007 that might be evidence of dark matter.

But it could be nothing at all.

CRAIG BENZINE: Now these are always-- these can go either way.

I mean, this could be the tip of an iceberg, or it could just be the background-- just a fluke, an accident.

MATT WEBER: So it's hard to detect dark matter, because it hardly ever interacts with regular matter.

But another problem is that we don't know exactly where all the dark matter is.

CRAIG BENZINE: If you want to have the best chance of finding dark matter, it's best to point your detector where all the dark matter is hiding.

And I'm very good at hide and seek.

I know these things.

So we're pretty sure our galaxy sits in a spherical halo of dark matter.

Most of the dark matter is in this halo.

And a surprisingly little amount of dark matter inhabits the disk where we live.

So looking for dark matter within our vicinity might be very difficult.

MATT WEBER: But we'd expect an increase in dark matter toward the core the galaxy since the density of matter increases.

CRAIG BENZINE: Of course, just because we know where dark matter might be doesn't mean we have any more of a clue of what it actually is.

The main theory is that dark matter is a new particle-- one that's not currently part of our standard model.

But there are other theories as well.

It's been theorized that dark matter isn't a particle at all, but that gravity might not work like we think it does.

On large scales, like the scale of galaxies and the universe at large, gravity might display different properties.

And the excess mass we see all over the universe might just be a side effect of this modified gravity.

But once you start messing with gravity, you start messing with relativity.

And everybody knows that Einstein doesn't go away without a fight.

MATT WEBER: So maybe it would better if dark matter wasn't some new, unheard of kind of matter, but regular matter, just in an exotic form, like a primordial black hole, or something even more undetectable than a neutrino.

CRAIG BENZINE: But that seems unlikely.

If it was ordinary matter and as common as it seems to be, it would start clumping up into visible objects like stars and galaxies.

MATT WEBER: Or maybe it's some kind of super fluid, which goes through different phases, which makes it hard to detect, and gives it unexpected behaviors.

CRAIG BENZINE: But the new particle theory is definitely the most widely accepted, and the strongest candidate for dark matter's secret identity.

MATT WEBER: And if it's one thing for sure, whatever dark matter's secret identity is, its discovery will revolutionize physics as we know it.

CRAIG BENZINE: And by revolutionize, we mean really screw up how we thought the universe works.

We have no idea what's going on.

MATT WEBER: Yeah, totally clueless.

RYAN: It seems like if there is another particle that makes up dark matter, it screws up this nice, neat, standard model that you have.

Is that, like, how you feel about that?

PADDY FOX: It would be awesome.

CRAIG BENZINE: Yeah, it would be awesome.

PADDY FOX: Yeah, the one thing every theorist, and I believe every experimentalist craves for and hopes for on a daily basis is something comes along that screws up the standard model.

The standard model is a wonderful theory that explains everything we've seen so far about subatomic physics, or subnuclear physics.

But we don't think it's the final story.

We don't think it is the final answer for the way Nature works.

So we believe there's more out there.

So to see some phenomena that isn't explained by the standard model would be fantastic.

And it could well be.

I mean, there's five times as much dark matter as there is regular matter.

So it's unlikely that the dark sector, as we like to call it, is simply just one new particle.

There's probably a whole lot of new phenomena in there that we just aren't privy to yet because it is hard for us to measure it.

But we need to find ways that will allow us to measure it.

And it will be a whole new realm of physics for us to investigate.

So what do you think dark matter is?

Is it a new particle, or is it gravity behaving weirdly?

Or maybe it's a wholly unknown form of physics that we're only beginning to understand.

And most importantly, will we ever be able to harness it as fuel for an interstellar delivery spaceship?

LEELA: It's dark matter.

BENDER: So this guy just unloaded a steaming pile of starship fuel?

Let us know in the comments.

I wanted to say that.

Let us know in the comments.

Go ahead, there you go.

I want to let you know about another series here on the internets that everyone on this show has worked on.

It's called-- Platoon of Power Squadron.

That's right.

It's about superheroes.

Well, no, they're not superheroes.

They're normal people with superpowers.

Thats' right.

And there's a star on that show who's awesome.

Yeah, Jake Jarvi.

No, Craig Benzine, me.

I'm in it as well.

Oh, you're in that show?

I am.

What part do you play?

Are you just like an extra or something?

Extra good.

If you've never heard of it, I'm going to put a link right up here on Matt's head that gives a great description of the first nine episodes.

And there's an Indie-Go-Go fundraiser linked in the du-be-du for the 10th episode.

After six years, the final episode is being made.

You guys should check it out.

It has nothing to do this show.

But we're all really proud of it, and we think you'll like it.

OK last week we talked about Henry Darger.

And you guys had a lot of things to say.

And we got a lot of things to say as well.

We do.

Bert Paulson, among others, pointed out that we didn't mention that some of the girls that Darger drew had penises.

This wasn't oversight.

And we did have some discussions about whether we should mention it or not.

But it seemed too distracting from the bigger picture.

And in the end, there's no explanation for it.

There's a lot of theories, though.

Some have mentioned that the penises represent Darger's issues of gender.

He might have been gay or experienced gender dysmorphia.

Or maybe he didn't understand the difference between girls or boys, or that the penises on the Vivian girls represented masculinity and power.

Whatever the reason, Darger isn't around to explain it to us.

So we will never know.

A few of you thought Darger's depictions of naked and tortured children represented a fetish, or maybe latent pedophilia.

And that's why he didn't want anybody to see his art.

We'll never know for sure what it all meant to Darger, but there doesn't seem to be much evidence that he was a pedophile.

He didn't sexualize the children.

And all the stories are from the point of view of the Vivian girls and the children.

So it seems like we're supposed to sympathize with them, and not the evil men that are torturing them.

There's much more evidence that Darger had a troubled childhood himself.

And the violence and the torture we see in his art is probably a way of him processing and dealing with his own child abuse.

And there's always the possibility that it means nothing at all.

He did it for no reason.

But we'll never know for sure.

I would also argue that there's a lot weirder and more disturbing art out there right now.

But we don't accuse the artists of being deranged, because they're still around to explain it.

The fact that we don't know much about Darger makes him kind of a blank canvas.

And we can make him whatever kind of person we want.

It's really up to you.

A lot of you had some great answers for what art is.

High-Waisted Pantaloon says "The Art Assignment"-- a fellow PBS digital show-- taught me that art has no single definition.

Art is everything.

Art is anything that makes you feel.

Verdatum said something similar.

He basically said, anything that evokes an emotional reaction could be considered art.

Sure.

LynneSkysong said that art doesn't need an audience.

Art just is.

I like that one.

I like the way you think.

Thanks for all your wonderful comments.

Next week is our final episode in the secrets playlist.

It's all about hackers.

Good hackers.

Yeah, good hackers.

Like Angelina Jolie and the other person.

In "Hackers"?

Yeah.

Would they be technically considered good hackers?

Yeah, they're the good guys, right?

In the movie.

I've never seen that movie.

Yeah, but they do break the law.

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