A cosmic detective story spanning a decade and a half has turned up an unexpected clue. The Fermi telescope, which scans the sky in gamma rays, has detected a steady, faint glow enveloping the entire Milky Way. It looks nothing like the light from stars or hot gas — more like a dim, all-pervasive haze. The brightest patch of this haze is not at the Galactic center but slightly above and below it, at high Galactic latitudes, and its energy points squarely to 20 gigaelectronvolts.
Two independent methods of data processing, averaging over bins and a finer pixel-by-pixel analysis, yield the same picture. The source appears as a giant sphere, compressed toward the Galactic center, which immediately rules out an extragalactic origin. The natural explanation that springs to mind is the annihilation of dark matter particles. When two such particles meet, they can transform into ordinary particles, producing a burst of gamma rays. Fitting the simplest model ($s$-wave annihilation) gives a particle mass of about 0.55 TeV for the $W^+W^-$ channel and 0.72 TeV for the $b\bar{b}$ channel — hundreds of times heavier than a proton.
But this beautiful hypothesis has an Achilles' heel. If dark matter particles annihilate so intensely in our Galaxy, then in dwarf spheroidal galaxies, which are also rich in dark matter, we should see an even brighter signal. Yet we do not — data from the same telescopes impose strict limits. This creates a tension of 4–5 times. The authors did not stop at acknowledging the contradiction; they tested alternatives. It turns out that the only way to save the hypothesis is to assume that annihilation is dramatically enhanced when particles collide at a specific, “resonant” velocity. This is like a swing that only picks up momentum when pushed in rhythm: in fast-moving systems (like dwarf galaxies), particles miss the resonance, but in our leisurely halo, they hit it. This mechanism, known as Sommerfeld enhancement or a Breit-Wigner peak, provides the necessary 45-fold boost in signal compared to a simple thermal relic.
The fine-tuning of the resonance to the specific velocities of our halo is a bold idea, but it does not contradict any observational data. If true, we are seeing not just circumstantial evidence but a concrete “portrait” of the dark matter particle, one that can be tested in future experiments.
The Fermi gamma-ray telescope, launched in 2008, was originally called GLAST and was renamed in honor of the Italian physicist Enrico Fermi, who in 1949 theoretically predicted the acceleration mechanism for cosmic particles that produce gamma rays.
In Peter Watts' novel Blindsight, an alien life form uses mechanisms sensitive to the velocity of particle interactions — much like the resonant annihilation of dark matter that tunes itself to the pace of motion in the galactic halo.