Fermi Searches for Dark Matter

Cosmological observations indicate that a large amount of invisible, non-baryonic matter (dark matter) may comprise most of the matter in the Universe. However, its fundamental properties remain a mystery searches using particle accelerators have failed to detect this weakly-interacting material. As such, the nature of dark matter remains one of the most important puzzles in modern science.

The simulated gamma-ray signal in Fermi-LAT from the brightest suspected dark matter annihilation targets such as the Galactic Center (brightest source) and its halo, along with dwarf spheroidal galaxies. + Learn More (Credit: Pieri, L. et al. 2011)

Why Fermi-LAT?

The Fermi Large Area Telescope (LAT) observes the whole gamma-ray sky from 0.3-300 GeV. Though designed to detect gamma rays, the LAT can also detect cosmic-ray electrons and positrons (CREs). Some dark matter candidates such as weakly interacting massive particles (WIMPs), are thought to have self-annihilation cross sections and/or decay rates that would allow them to produce detectable particles, including gamma rays and CREs in the sensitivity range of Fermi-LAT. With its excellent sky coverage and sensitivity, Fermi-LAT provides a unique platform for indirect dark matter searches by surveying a wide variety of astrophysical sources and probing astrophysical processes that could reveal clues to the nature of dark matter. + Learn More

Theorists have speculated that the strong gravitational effects caused by the presence of dark matter may mean these particles are very massive. If so, their self-annihilation would likely result in a signature visible in the gamma-ray regime.

Dwarf Spheroidal Galaxy Searches

Dwarf spheroidal galaxies (dSphs) are considered to be excellent targets for indirect dark matter searches as they are believed to be dominated by dark matter and not contain a significant population of known gamma-ray sources. A study combining observations taken on 25 dSphs has provided some of the most constraining upper limits on the thermally averaged self-annihilation cross section of WIMP dark matter. + Learn More

The limits on the thermally averaged annihilation cross section of dark matter as a function of energy. The different graphs represent various annihilation channels. (Credit: Fermi-LAT collab)

An Excess At The Galactic Center?

The Galactic Center is predicted to be the closest, brightest source of gamma rays from dark matter annihilation. Analysis of the halo of the Galactic Center with Fermi-LAT (subtracting the diffuse emission from the region) by gamma-ray researchers has revealed a GeV excess near the Galactic Center. This excess has an energy spectrum that peaks at several GeV and is approximately spherically symmetric. Both of these features are consistent with expectations from a dark matter signal from the Galactic Center. While tantalizing, it is possible that this signal is instead due to conventional astrophysics (such as a population of millisecond pulsars) or an incomplete understanding of the subtracted background in the region. Additionally, a GeV excess at the Galactic Center with the observed properties is in tension with non-detections made of other dark matter targets (such as dwarf spheroidal galaxies). Future observations of both the Galactic Center region, as well as of dSphs, may provide additional information about the source of this excess.

The total Fermi-LAT gamma-ray excess map with an inset showing the apparent GeV excess surrounding the Galactic Center. (Credit: NASA/T. Linden, U.Chicago) + Learn More

Additional Dark Matter Targets

The hunt for dark matter continues with Fermi-LAT across a wide variety of astrophysical sources. Here are some additional targets used by Fermi-LAT scientists to search for the elusive gamma-ray signal indicative of the presence of particle dark matter.

Galaxy Clusters

Known to be the largest gravitationally bound structures in the universe, galaxy clusters are dominated by dark matter making them excellent targets for searches with Fermi-LAT. Current searches for gamma rays from nearby clusters have yet to reveal a significant gamma-ray signal. + Learn More

Abell 2744, one of the most massive galaxy clusters known. Clusters such as this are ideal targets for Fermi-LAT dark matter searches. (Credit: JPL/Hubble)

Gamma-ray Background

Dark matter annihilation, both locally and integrated over the history of the universe should produce a deviation in the expected diffuse gamma-ray background in Fermi-LAT observations. Careful examination of the spectral characteristics of the gamma-ray background within Fermi-LAT has already produced constraining limits on dark matter annihilation and will continue to do so over the lifetime of the mission. + Learn More

Upper limits on the dark matter cross section derived from the extragalactic gamma-ray background energy spectrum. Known sources can account for almost all the background radiation, leaving very little room for any contribution from dark matter. + Learn More