A city at night from above: bright streetlights, glowing windows, car headlights. Astronomers see the center of our Galaxy in much the same way — only not in ordinary light but in gamma rays. And there, among the familiar lights, a strange, uniform glow emerges. It cannot be attributed to any known source. This excess is called the Galactic-center excess (GCE).
A new analysis of Fermi data has worked like replacing a frosted window with a clear one. Previously, the picture was blurred due to inaccurate models of the interstellar medium and background sources. Now scientists applied a multi-step fitting and iterative source finding. Errors have been reduced to less than 10% in a 40×40 degree region around the center. The glow profile turns out to be very compact and nearly spherical. Brightness falls off smoothly from the center. This fits poorly with the idea that the light comes from a scattering of millisecond pulsars — rapidly rotating neutron stars. Such stars should trace the shape of the stellar bulge (the elongated bump at the Galaxy's center).
But the spherical shape and spectrum match well the predictions for annihilation of dark matter particles. Of course, this is not proof. But now we have a much clearer target for testing. We are not just seeing a mysterious light; we are beginning to discern its outlines. It's as if the night city gradually reveals the map of its hidden neighborhoods.
Gamma rays from the Galactic center take about 25,000 years to reach us, and during that time the core of the Milky Way itself changes noticeably.
In Peter Watts's novel Blindsight, mysterious signals from space also turn out to be not what they seem, and the attempt to explain them leads to terrifying discoveries.