At the very heart of the Milky Way, something is glowing, but not with starlight; with gamma rays invisible to the eye. Telescopes have been catching this excess for over a decade, and astrophysicists argue: is it the fault of rapidly spinning dead stars, pulsars, or do dark matter particles collide and vanish in a flash? New work proposes a third path, akin to a game of billiards.
Cosmic rays, mostly protons, fly through the galaxy at nearly the speed of light. If a dark matter particle crosses their path, the impact might not be head-on but glancing: the proton transfers energy, the dark particle gets excited, and then emits two photons. Or, in another variant, a photon is born right at the moment of collision, like a spark from striking flint against flint. The authors calculated that such a 'billiard' exactly reproduces the shape of the gamma-ray spectrum that instruments see.
The model requires no exotic assumptions. It's enough that dark matter consists of two particles with similar masses, or that the collision produces an extra photon. Both cases fit within known theories and yield predictions for future observations. If the idea is correct, the galactic center is not a graveyard of dark particles but an arena where they constantly 'ring' under the blows of cosmic rays.
as written by the authors arXiv:2605.08010
In this work, we propose a novel mechanism for generating gamma rays from the Galactic Center via scattering of cosmic-ray protons off dark matter in the Milky Way halo, in contrast to conventional explanations based on dark matter annihilation. We present two examples of this framework that produce an observable photon signal. In the inelastic dark matter model, cosmic rays up-scatter a lighter dark matter particle, with the subsequent decay of the heavier particle yielding two photons. In the elastic dark matter model, an energetic photon is directly produced in the final state of a 2-to-3 scattering process. We show that, for a range of viable model parameters, this framework provides a fit to the observed Galactic Center gamma-ray excess spectrum comparable to those obtained from dark matter annihilation and millisecond pulsar models. Our results open a new avenue for interpreting gamma-ray observations of the Galactic Center.
The energy of a single gamma photon from such a collision can be billions of times greater than the energy of visible light — roughly as if a mosquito bite carried the force of a falling brick.
In the novel 'Darkness Beyond the Edge of the World,' invisible particles react to stellar radiation, creating a ghostly glow — much like the proposed mechanism.