Dark matter remains one of the greatest mysteries in the universe, an invisible substance thought to make up five-sixths of all matter in the universe. Research teams around the world are striving to discover what exactly dark matter is by looking at gamma rays it might generate, and scientists may be on the verge of a discovery. The work is reviewed in the Proceedings of the National Academy of Sciences.
Dark matter is apparently invisible and nearly completely intangible, mostly only detectable via the gravitational pull it exerts. The scientific consensus right now is that dark matter is composed of a new type of particle, one that interacts very weakly at best with all the known forces of the universe, except gravity.
“One of the most puzzling questions in present-day cosmology and particle physics is, ‘What is dark matter made of?'” asks particle astrophysicist Stefan Funk at the SLAC National Accelerator Laboratory in Stanford, California.
Assuming that dark matter is composed of weakly interacting massive particles (WIMPs), some models suggest these particles could give off gamma rays when they decay or when they collide with and annihilate each other. The gamma rays scientists detect in the universe could therefore reveal whether dark matter particles actually exist, and what their properties might be, such as mass and the cross-sections, or the way in which they interact with other particles.
Scientists expect WIMP masses to range from 10 billion electronvolts (GeV) to a few trillion electronvolts (TeV). (In comparison, a proton is only 938 million electronvolts in mass.) Since Einstein’s famous equation E=mc2 revealed that mass and energy are equivalent to each other, researchers can search for potential signs of WIMPs by looking for gamma rays with GeV and TeV energies.
Gamma rays have several unique properties that make them ideally suited to study dark matter annihilations, Funk said. For example, they are not deflected by magnetic fields, thus meaning they can serve as signposts back to wherever they were created, helping reveal the distribution of dark matter in the universe.
One ideal type of place to focus searches are dwarf spheroidal galaxies, where star formation is usually highly suppressed, making the gravitational signature of dark matter easy to model. Currently, there are roughly 25 known dwarf satellite galaxies to the Milky Way, and both ground-based instruments such as the High-Energy Stereoscopic System (HESS), Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC), and Very Energetic Radiation Imaging Telescope Array (VERITAS) as well as the Fermi-LAT (Large Area Telescope) are actively observing these objects. Analysis of data from these telescopes has already helped constrain models about dark matter.
Galaxy clusters are the largest massive objects in the universe, and like dwarf spheroidals, they are probably dark-matter dominated. One complication for using them in searches for dark matter is that galaxy clusters also often contain a significant number of astrophysical sources of gamma rays, such as supermassive black holes.
Fermi-LAT has also measured the so-called isotropic gamma-ray background, the faint diffuse gamma rays from all over the sky. Although it would be extremely difficult to tell which if any of these came from dark matter, it has helped set upper bounds on dark matter annihilation rates.
The galactic center is expected to be the brightest source of gamma rays from dark matter annihilation by at least two orders of magnitude. However, gamma rays from there may also emerge from a multitude of astrophysical sources, as well as from cosmic rays interacting with dense molecular clouds in the inner galaxy. Data from Fermi-LAT have been used to search for gamma rays resulting from dark matter annihilation in the galactic dark matter halo and the central part of the galaxy.
Funk notes the smoking gun signature of dark matter will be gamma rays all of very specific wavelengths, a signal that is very difficult to mimic using astrophysical sources. Recently hopes were raised that such a signal may have been detected — gamma rays of 130 GeV and a very specific line of wavelengths in an extended region around the galactic center might be the long-awaited first clear evidence of dark matter annihilation into gamma rays. However, Funk notes that over time, this 130-GeV finding appears to have been a combination of statistical fluke and a systematic problem in the detector.
The dark matter community is now looking toward the next generation of ground-based instruments as the next big step toward the indirect detection of dark matter through gamma rays. The Cherenkov Telescope Array (CTA) is expected to start operation later this decade and will have sensitivity over the energy range from a few tens of GeV to hundreds of TeV.
“We might be on the verge of a discovery,” Funk says. “The idea that we might be detecting dark matter either in an accelerator like the LHC at CERN, directly in an underground lab, or indirectly through gamma-ray emission is very exciting.”