After decades of careful searching, astronomers and physicists are finally closing in on dark matter—the unseen majority of stuff in the universe.
Physicists first theorized in the 1930s that a mysterious form of matter was holding galaxies together. By the early 1980s, researchers began sharing ideas about what kind of properties those hidden particles might have. Recently, many instruments designed to hunt for dark matter have come online (including those in the gallery above), and scientists are finally beginning to see results emerge—as evidenced by the bevy of dark matter announcements and press in the last few weeks.
Some of the observations and claims from those instruments have been genuinely thrilling, hinting at the type of particle (or particles) that actually makes up the dark stuff; others have been dubious, to put it kindly. Let us separate the hope from the hype for you.
Dark Forces at Work: Potentially Revolutionary
In the Abell 3827 galaxy cluster, visible from the Southern Hemisphere, a group of faraway galaxies are colliding in a grand cosmic pileup. Last week, astronomers looking carefully at those collisions published that curiously, the dark matter in those galaxies seems to be interacting with itself. That’s a surprise, because dark matter has built a reputation for being almost maddeningly shy. (It doesn’t seem to interact with anything, except for gravity. It doesn’t reflect light. It doesn’t feel magnetism. In fact, it doesn’t seem to care very much about us at all.)
Spying on the galaxies in Abell 3827 with the European Southern Observatory’s Very Large Telescope (seriously, that’s its name) and NASA’s Hubble Space Telescope, astronomer Richard Massey and colleagues saw that dark matter—detectable through gravitational lensing—seems to be lagging behind, decoupled from its home galaxy as if it were sticking to something. That means that the dark matter might be feeling its own force (or forces), a refinement of a similar observation made in the same cluster in 2010 by Liliya Williams and Prasenjit Saha, both of whom worked on the new study.
For Massey, who has spent his career tracking dark matter, the observation feels like validation. “After chasing something for so long,” he says, “it’s really nice to have finally caught it in the act of doing something.” Any hint of activity could help physicists nail down exactly what dark matter really is. (The group’s last news release, in late March, was less exciting: An examination of 72 galactic cluster collisions using Hubble and NASA’s Chandra X-ray Observatory revealed hardly any evidence that dark matter interacted with itself.)
There is something very special about Abell 3827, also called the Bullet Cluster, says Joel Primack, one of the formers of the dominant model for dark matter. Astronomers have observed a lot of cluster collisions, but there’s something unique about the angle and position of this one that allowed dark matter’s stickiness to stick out. “No one’s ever seen such a thing before, with clarity and precision, except in this cluster,” says Primack. It’s too early to call this a discovery, he says, “but they’ve done a beautiful job analyzing images, and this could be very important—potentially revolutionary.”
(What it is not is evidence that dark matter is somehow “less dark” than we thought, as has been reported elsewhere. Boo on bad press release headlines—I’m talking to you, ESO.)
Southern Map Maker: Old Reliable
In Chile, atop a mountain, one of the world’s most powerful digital cameras (570-megapixels) is bolted to Victor M. Blanco Telescope, giving scientists in the Dark Energy Survey collaboration an unprecedented view of the southern skies. Last week they released their first map detailing the presence of dark matter across a large region of previously-unmapped sky. This is just the first of many maps they’ll release over their five-year mission.
The project doesn’t have any grand aspirations to define dark matter—it’s just a mapping project. Astronomers have made dark matter maps before, and the observations from DES jive perfectly with how they already understand dark matter to behave. But that doesn’t mean the survey’s results are worthless: Many researchers, including Primack, are excited to follow the work of DES as it illuminates new areas of the sky with greater precision than ever before.
Spectro-Speculation: Maybe Hogwash
Primack doesn’t feel the same way about the Alpha Magnetic Spectrometer, an instrument aboard the International Space Station reading cosmic rays, high-energy radiation that’s constantly zipping through space. AMS shared their latest results last week, saying they had detected more antiprotons in the cosmic radiation than they expected. In some models dark matter can form an excess of antiprotons—meaning astronomers might be able to detect dark matter by detecting that radiation.
Although AMS has made a lot of noise about their antiproton findings, they already fit within standard model physics, says Stefano Profumo, a theoretical physicist at the Santa Cruz Institute for Particle Physics. The radiation detected by the AMS could just as easily arise from non-dark matter sources.
“There is absolutely no reason to think AMS results have anything to do with dark matter or new physics,” Profumo says. Primack goes further: “This is basically a stupid instrument.” It’s nice to get more data about the physics of cosmic rays, he qualifies, “but is it teaching us anything new? The answer is no.” AMS didn’t respond in time to a request for comment.
Experimenters are eager, and occasionally desperate, to share relevant discoveries. Yet detecting dark matter is such a difficult task, says Katie Mack, a postdoctoral astrophysicist at the University of Melbourne and columnist at Cosmos magazine, that astronomers should greet each claim of dark matter detection with a skeptical eye.
When you’re trying to find a signal, she says, “you have to contend with all of the astrophysical processes in the universe, some of which we can’t see easily or which might produce weird effects. And you have to convince yourself that some strange excess that you found is not to do with anything in known—or not-yet-known—non-dark matter astrophysics. That’s a big ask.”
Awesome Stuff on the Horizon
All the same, these are extremely exciting times for fans of dark matter. The Large Hadron Collider, which just restarted, might be on the cusp of making dark matter. “So, we’re hopeful,” says Mack. “And this is one of the things that would make these indirect detections much more compelling—if you can create something in the lab that is consistent with a sky signal.”
You bet scientists are entering a special time for dark matter, says Primack, but it’s been a long time coming. “Everyone in the field knows we’re going to discover what dark matter is before too long,” he says. “And considering that dark matter is responsible for most of the mass in the universe, it’s not a minor detail.”
