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.