The Galactic Center excess is an unexpected concentration of gamma-rays emerging from the center of our Milky Way Galaxy.
This view shows the entire sky at energies greater than 1 GeV based on five years of data from the LAT instrument on NASA’s Fermi Gamma-ray Space Telescope; the most prominent feature is the bright band of diffuse glow along the map’s center, which marks the central plane of our Milky Way Galaxy. Image credit: NASA / DOE / Fermi LAT Collaboration.
Gamma-rays are a form of electromagnetic radiation with the shortest wavelength and highest energy.
The anomalous gamma-ray signal coming from the core of the Milky Way Galaxy was first detected in 2009 by the Large Area Telescope, the primary instrument on NASA’s Fermi Gamma-ray Space Telescope.
Its origin has been debated, with proposed sources prominently including self-annihilating dark matter and an undetected population of millisecond pulsars.
“When Fermi pointed to the Galactic center, the results were startling,” said Dr. Noam Libeskind, an astrophysicist at the Leibniz Institute for Astrophysics Potsdam.
“The telescope measured too many gamma rays, the most energetic kind of light in the Universe.”
“Astronomers around the world were puzzled, and competing theories started pouring in to explain the so-called gamma-ray excess.”
“After much debate, two ideas rose to the fore: either these gamma rays were the result of millisecond pulsars (ultra-dense neutron stars that spin thousands of times per second) or from dark matter particles smashing into each other and annihilating. Both theories have their drawbacks.”
“However, our results effectively confirm the theory that the gamma-ray excess is due to dark matter annihilation.”
In their study, Dr. Libeskind and colleagues modeled the formation of Milky Way-like galaxies under environmental conditions similar to those of Earth’s cosmic neighborhood.
They found that dark matter does not radiate outwards from the Galactic center, but is instead organized similar to that of stars, meaning the former could just as equally have produced the excess gamma rays.
“The Milky Way galaxy has long been known to live in a so-called dark matter halo, a spherical region filled with dark matter around it,” said Dr. Moorits Mihkel Muru, an astrophysicist at the Leibniz Institute for Astrophysics Potsdam and the University of Tartu.
“However, the extent to which this halo is aspherical or ellipsoidal has not been appreciated.”
“We analyzed simulations of the Milky Way and its dark matter halo and found that the flattening of this region is sufficient to explain the gamma ray excess as being due to dark matter particles self-annihilating.”
“These calculations demonstrate that the hunt for dark matter particles — that can self-annihilate — should be encouraged and bring us one step closer to understanding of mysterious nature of these particles.”
A paper on the findings was published this month in the journal Physical Review Letters.
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Moorits Mihkel Muru et al. 2025. Fermi-LAT Galactic Center Excess Morphology of Dark Matter in Simulations of the Milky Way Galaxy. Phys. Rev. Lett 135, 161005; doi: 10.1103/g9qz-h8wd
