So far, dark matter has evaded scientists’ best attempts to find it. Astronomers know the invisible stuff dominates our universe and tugs gravitationally on regular matter, but they do not know what it is made of. Since 2009, however, suspicious gamma-ray light radiating from the Milky Way’s core—where dark matter is thought to be especially dense—has intrigued researchers. Some wonder if the rays might have been emitted in explosions caused by colliding particles of dark matter. Now a new gamma-ray signal, in combination with those already detected, offers further evidence that this might be the case.
One possible explanation for dark matter is that it is made of theorized “weakly interacting massive particles,” or WIMPs. Every WIMP is thought to be both matter and antimatter, so when two of them meet they should annihilate on contact, as matter and antimatter do. These blasts would create gamma-ray light, which is what astronomers see in abundance at the center of our galaxy in data from the Fermi Gamma-Ray Space Telescope. The explosions could also create cosmic-ray particles—high-energy electrons and positrons (the antimatter counterparts of electrons)—which would then speed out from the heart of the Milky Way and sometimes collide with particles of starlight, giving them a boost of energy that would bump them up into the gamma-ray range. For the first time scientists have now detected light that matches predictions for this second process, called inverse Compton scattering, which should produce gamma rays that are more spread out over space and come in a different range of energies than those released directly by dark matter annihilation.
“It looks pretty clear from their work that an additional inverse Compton component of gamma rays is present,” says Dan Hooper, an astrophysicist at the Fermi National Accelerator Laboratory who was not involved in the study, but who originally pointed out that a dark matter signal might be present in the Fermi telescope data. “Such a component could come from the same dark matter that makes the primary gamma-ray signal we've been talking about all of these years.” University of California, Irvine scientists Anna Kwa and Kevork Abazajian presented the new study October 23 at the Fifth International Fermi Symposium in Nagoya, Japan and submitted their paper to Physical Review Letters.
None of the intriguing gamma-ray light is a smoking gun for dark matter. Other astrophysical processes, such as spinning stars called pulsars, can create both types of signal. “You can make models that replicate all this with astrophysics,” Abazajian says. “But the case for dark matter is the easiest, and there’s more and more evidence that keeps piling up.”
The official Fermi telescope team has long been cautious about drawing conclusions on dark matter from their data. But at last week’s symposium, the group presented its own analysis of the unexplained gamma-ray light and concluded that although multiple hypotheses fit the data, dark matter fits best. “That’s huge news because it’s the first time they’ve acknowledged that,” Abazajian says. Simona Murgia, an astrophysicist at the University of California, Irvine and a member of the Fermi collaboration’s galactic-center analysis team, presented the team’s findings. She says the complexity of the galactic center makes it difficult to know for sure how the excess of gamma rays arose and whether or not the light could come from mundane “background” sources. “It is a very interesting claim,” she says of Abazajian’s analysis. “However, detection of extended excesses in this region of the sky is complicated by our incomplete understanding of the background.”
The dark matter interpretation would look more likely if astronomers could find similar evidence of WIMP annihilation in other galaxies, such as the two dozen or so dwarf galaxies that orbit the Milky Way. “Extraordinary claims require extraordinary evidence, and I think a convincing claim of discovery would probably require a corresponding signal in another location—or by a non-astrophysical experiment—as well as the galactic center,” says Massachusetts Institute of Technology astrophysicist Tracy Slatyer, who has also studied the Fermi data from the Milky Way’s center.
Non-astrophysical experiments include the handful of so-called direct-detection experiments on Earth, which aim to catch WIMPs on the extremely rare occasions when they bump into atoms of normal matter. So far, however, none of these has found any evidence for dark matter. Instead they have steadily whittled away at the tally of possible types of WIMPs that could exist.
Other orbiting experiments, such as the Alpha Magnetic Spectrometer (AMS) on the International Space Station, which detects cosmic rays, have also failed to find convincing proof of dark matter. In fact, the AMS results seem to conflict with the most basic explanations linking dark matter to the Fermi observations. “Most people would agree that there is something rather unexpected happening at the galactic center, and it would be tremendously exciting if it turns out to be a dark matter annihilation signal,” says Christoph Weniger of the University of Amsterdam, another astrophysicist who has studied the Milky Way’s core. “But we have to confirm this interpretation by finding corroborating evidence in other independent observations first. Much more work needs to be done.”