Dark matter is a riddle: Stars can feel it, but we can’t see it. It makes up 80% of the matter in the universe, yet we can only guess at what it actually is.
The puzzle began in 1933, when the Swiss astronomer Fritz Zwicky pointed out that galaxies in a group called the Coma Cluster seemed to be moving too quickly to be bound together by the cluster’s gravity. Stronger evidence followed in the 1960s and 1970s, when Vera Rubin’s studies of galactic rotation revealed that stars at the outer edge of spiral galaxies were rotating just as fast as those in the center. That extra speed should have sent them flying out into intergalactic space. Gravity held them back—but gravity from what? When astronomers estimated the total mass contained in all the stars, gas, and dust in the galaxies, it didn’t add up: Either the laws of gravity were wrong, or some invisible “dark matter” was hiding in the galaxies’ dim margins.
Since those early observations, astronomers have found a wealth of indirect evidence for dark matter. Dark matter bends light from distant galaxies and shapes and distorts the cosmic microwave background radiation. “It is like seeing a dancing marionette puppet (the gravitational effects), and inferring there must be strings (dark matter) to make it move,” wrote Ben Cook, an astrophysics graduate student at the Harvard-Smithsonian Center for Astrophysics, on Astrobites. “To prove dark matter really exists, we must actually see the strings, not just the puppet.”
Today, the leading theory is that dark matter is made up of subatomic particles called weakly interacting massive particles (WIMPs), relatively heavy particles that only feel gravity and a fundamental force called the weak force. Because they don’t interact with ordinary matter via the other fundamental forces, they can pass right through walls, people, even entire planets, with just a tiny chance of actually getting close enough to a nucleus of regular matter to disturb it.
But, physicists still haven’t found any direct evidence of WIMPs, or any other kind of dark matter. No one has definitively caught a dark matter particle or spotted it doing anything other than, well, gravitating—that is, invisibly exerting a force on the ordinary matter around it. It’s certainly not for lack of trying. Using detectors like the Large Underground Xenon (LUX) and Cryogenic Dark Matter Search experiments, physicists have been trying to catch the subatomic slipstreams left behind by cosmic dark matter particles, and at accelerators like the LHC, they are trying to create dark matter from scratch. At the same time, astrophysicists are searching the skies for the gamma-ray lightshows that could come from dark matter particles crashing into and annihilating each other.
Now, astrophysicists may finally be closing in on dark matter. In 2009, researchers Lisa Goodenough and Dan Hooper found surprisingly bright gamma ray emission coming from the center of the Milky Way. Right away, they noticed that the emission was “bumpy,” says Hooper, meaning that it was concentrated within a narrow energy band rather than being smoothly spread out across the gamma-ray spectrum. The peaked spectrum was unusual for known gamma ray sources, but it squared up well with predictions for dark matter annihilation. While Hooper and others continue to consider other possible explanations for these homegrown gamma rays—a mob of exotic pulsars, an outburst from our central supermassive black hole—Hooper thinks dark matter is the likeliest culprit. “It’s pretty easy to explain with dark matter, and pretty difficult to explain with other stuff,” says Hooper.
When it comes to picking up gamma ray signals, the galactic center has the advantage of proximity. But, “things are notoriously messy and complicated” in there, says Hooper. So researchers have turned their attention to dwarf galaxies, which are rich in dark matter and not much else. “The ratio of dark matter to ordinary matter in dwarfs is very, very high,” explains Hooper; dwarf galaxies may contain thousands of dark matter particles for every one ordinary particle. Otherwise placid, they make ideal hunting grounds for unusual gamma ray emission.
“Dwarfs are what everyone’s got their eye on,” says Hooper. But, though they are probably the commonest galaxy type in the cosmos, they are difficult to find, and a recent search for excess gamma ray emission from the few known nearby dwarfs came up empty-handed.
Now, that story may be changing. Combing through data from the Fermi spacecraft’s Large Area Telescope (LAT), two groups of researchers found surprisingly bright gamma-ray emission coming from a newly-discovered dwarf galaxy called Reticulum 2. While the teams can’t rule out the possibility that the gamma rays are coming from a more mundane source or that they’re random background noise, they think that the gamma-ray signal matches what you’d expect from dark matter particles crashing into and annihilating each other.
A third analysis, which was based on nearly identical raw data processed with a newer algorithm, turned up more gamma ray photons than expected, but not enough to make the case for a meaningful signal. It isn’t clear yet whether these apparently clashing results are truly contradictory, says Alex Geringer-Sameth of Carnegie Mellon University, lead author on the first paper to claim a detection of excess gamma rays. Once the new data set, called “Pass 8,” is made public, researchers like Hooper and Geringer-Sameth will comb through it trying to understand the source of the disagreement.
If the signal is real (“a HUGE if” emphasizes Garinger-Sameth), it will be a major advance for particle physics. Finally, physicists will be able to get a fix on dark matter’s mass and its annihilation process. “If the Reticulum 2 signal holds up, other groups will have a specific target to shoot for,” said Geringer-Sameth in an email. “Imagine an underground detector finding new particles streaming through the Earth that have the same properties as the new particles a hundred thousand light years away in a dwarf galaxy. It would be a very strong case that what both groups are seeing is actually the dark matter.”
If the observation does not hold up, though, astrophysicists will have a fresh puzzle on their hands. Fermilab physicist Alex Drlica-Wagner, who led up the team that measured the smallest gamma-ray excess, points out that a succession of gamma-ray no-shows from dwarf galaxies could scuttle the notion that galactic center gamma rays are really due to dark matter. “If the excess emission from galactic center comes from dark matter annihilation, then we should start seeing a similar gamma-ray signal coming from the dwarf galaxies. So far, we have not,” he wrote in an email.
“I believe that the most important question to answer in this analysis is, ‘How much dark matter is in Reticulum 2?’” says Drlica-Wagner. To find out, astronomers will be tracking the motion of stars within Reticulum 2 with optical telescopes to reveal how their velocity is being affected by dark matter’s gravity.
Reticulum 2 is one of nine dwarf galaxies just discovered by the Dark Energy Survey (DES) and is still the only dwarf galaxy from which astronomers have detected a gamma-ray excess. “Before 2006 there were only about 10 dwarf companions of the Milky Way that were known,” says Geringer-Sameth. Now, we are on the brink of a “tsunami” of new discoveries, he says, with data flooding in from the DES and a sky survey called the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS). In the next decade our census of dwarf galaxies may triple, Hooper estimates: “That could totally change the game.”
As new dwarf discoveries come in, astrophysicists can immediately begin searching for their gamma-ray counterparts in Fermi’s rich data archive. “We have data,” says Geringer-Sameth. “We just don’t know where to look.”
Meanwhile, Hooper and his colleagues continue to analyze gamma rays coming from the galactic center. The first study, back in 2009, was based on a single year of data from the Fermi spacecraft; now Hooper has six and a half years of measurements to work with, and he and his team can perform a more discriminating analysis of the data. Last year, a team that included Hooper and a half-dozen colleagues from around the country showed that the spectrum’s “bumpy” shape was consistent across the galactic center, and that the emission got progressively brighter closing in on the heart of the galaxy, just as predicted. Again and again, says Hooper, it “passes the test.”
Will the gamma ray signals stand up to the next round of scrutiny? We’ll have to wait to find out if we’re finally seeing the light at the end of the dark matter tunnel—or if there are still unknown miles of darkness ahead.
Editor’s picks for further reading
The Nature of Reality: Journey Into the Dark Realm
What if dark matter isn’t just one particle, but a diverse realm of dark matter particles that experience forces that don’t affect ordinary matter? Don Lincoln explores the “dark realm.”
NOVA Next: Pursuing Dark Matter, Hundreds of Miles Above Earth
The Alpha Magnetic Spectrometer, aboard the International Space Station, has been searching for evidence of dark matter since 2011. Veronique Greenwood reports on what it has found.
NOVA scienceNOW: Dark Matter
In this video, Neil deGrasse Tyson reports from a half mile underground in an abandoned mine, where scientists are using special detectors to look for dark matter particles.
The New York Times: Gamma Rays May Be Clue on Dark Matter
Dennis Overbye reports on new gamma ray studies of Reticulum 2.