The Galactic center teems with gamma-ray sources, from interacting binary systems and isolated pulsars to supernova remnants and particles colliding with interstellar gas. It's also where astronomers expect to find the galaxy's highest density of dark matter, which only affects normal matter and radiation through its gravity. Large amounts of dark matter attract normal matter, building a foundation upon which visible structures, like galaxies, form.
No one knows the true nature of dark matter, but WIMPs, or Weakly Interacting Massive Particles, represent a leading class of candidates. Theorists have envisioned a wide range of WIMP types, some of which may mutually annihilate when they collide and directly produce photons or intermediate, quickly decaying particles. Both of these pathways end with the production of gamma-rays -- the most energetic form of light -- at energies within the detection range of Fermi's Large Area Telescope (LAT).
When astronomers carefully subtract all known gamma-ray sources from LAT observations of the Galactic center, a patch of leftover emission remains. This excess appears most prominent at energies between 1 and 3 billion electron volts (GeV) -- roughly a billion times greater than that of visible light -- and extends outward at least 5,000 light-years from the Galactic center.