Has the universe’s most elusive substance finally betrayed its presence? A new analysis of fifteen years of data from NASA’s Fermi Gamma-ray Space Telescope has turned up a faint, spherical glow of high-energy gamma rays surrounding the Milky Way-radiation that closely matches the predicted signature of dark matter annihilation. Confirmation of this would amount to a first direct detection of dark matter, the breakthrough physicists have pursued for almost a century.
It was observed by Tomonori Totani, an astrophysicist at the University of Tokyo, who focused his search on the galactic halo rather than the crowded galactic center. The former is the region above and below the disk of the Milky Way and is deemed to contain copious amounts of dark matter but comparatively few astrophysical sources of gamma rays-a cleaner laboratory for detection. He modeled and subtracted known sources of emission in the region: the enormous Fermi bubbles, cosmic-ray interactions with interstellar gas, and point sources; what was left was a residual component peaking sharply at 20 GeV, the energy long predicted for WIMP annihilation.
WIMPs are considered among the foremost candidates for dark matter – particles much heavier than the proton and which interact so weakly with normal matter that their direct detection is beyond reach. Indeed, theory predicts that two colliding WIMPs annihilate in a shower of particles including gamma-ray photons. The energy spectrum for the observed emission in Totani’s analysis is consistent with that expected from WIMPs with masses roughly 500 times greater than that of a proton, while the annihilation rate implied is consistent, within the uncertainties, with theoretical predictions. Finally, the radial profile of this emission is consistent with the NFW profile conventionally used to describe galactic dark-matter halos.
Most of this work is based on the Fermi LAT, which is designed to detect photons in an energy range from 20 million electronvolts to hundreds of gigaelectronvolts with a wide field of view and high angular resolution. The LAT’s very long operational life has finally made it possible to collect the enormously large dataset required for extracting weak signals out of noisy backgrounds. The faintness of the halo called for statistical processing of billions of detected photons just to lift the signal above the noise floor in the case at hand.
But scepticism still prevails. Astrophysical mechanisms-from pulsars to supernova remnants-can also produce gamma rays at similar energies. “Because the backgrounds of astrophysical sources are so uncertain, it makes it very difficult to make strong claims,” says Jan Conrad, an astroparticle physicist at Stockholm University. Previous excesses of gamma rays, such as the long-debated galactic centre signal, have resisted definitive attribution to dark matter for over a decade.
Independent verification is essential. One fruitful direction in particular is that of searching for matching gamma-ray spectra from nearby dwarf spheroidal galaxies orbiting the Milky Way. These galaxies are rich in dark matter, and crucially, they have particularly low gamma-ray backgrounds. In this respect, they form ideal testbeds in which this search may be carried out. The fact that such signals have not been seen up until now has formed the major counter-argument against interpreting these galactic excesses as arising due to dark matter. Totani agrees that “the decisive factor” is to find matching emissions from these cleaner environments.
Future instrumentation could provide decisive evidence. The forthcoming CTAO will dramatically improve sensitivity to high-energy gamma rays, allowing far more precise mapping of the halo signal. In parallel, underground direct-detection experiments-such as the DarkSide-20k liquid-argon detector-seek to capture nuclear recoils from WIMPs of the same mass range implied by the Fermi halo. A concordant signal from both astrophysical and terrestrial detectors would be a strong case for dark matter.
The fact that Totani’s excess survived so many background models and systematic checks guarantees its status as one of the most compelling dark matter candidates so far. As Kinwah Wu from University College London summed it, “we need extraordinary evidence for an extraordinary claim.” The history of dark matter searches is full of promising signals that were later ascribed either to conventional astrophysics or to analysis artefacts. For the time being, this 20 GeV gamma-ray halo remains a tantalizing clue-one that might, with cross-verification, shed light on the invisible scaffolding of the cosmos.
