Building on prior detections of gamma-ray emissions from the Milky Way's center, physicists led by Gordan Krnjaic at Fermilab propose dark matter consists of two distinct particles that interact to produce detectable signals. This resolves the puzzle of signals in the Milky Way but none in dark-matter-rich dwarf galaxies, as observed by the Fermi Gamma-ray Space Telescope.
Previous analyses, such as a 2025 study by Tomonori Totani using Fermi data, identified an excess of gamma rays forming a halo-like structure toward the Milky Way's center, potentially from dark matter particle annihilation. However, no corresponding signals have been detected in nearby dwarf galaxies, which are rich in dark matter but low in astrophysical background noise—a challenge for standard single-particle dark matter models.
In a new study, Gordan Krnjaic, a theoretical physicist at Fermilab, and collaborators suggest dark matter comprises two types of particles that only produce gamma rays when they interact with each other. Krnjaic explained the observation: 'Right now there seems to be an excess of photons coming from an approximately spherical region surrounding the disk of the Milky Way.' He added, 'If certain theories of dark matter are true, we should see it in every galaxy, for example in every dwarf galaxy.'
This two-particle model accounts for the Milky Way signal due to sufficient density of both components there, while dwarf galaxies lack one type, preventing signals. The proposal reconciles Fermi observations without contradicting other data.