Scientists have been analyzing high-energy gamma rays originating from the center of the Milky Way and have presented the most convincing case so far that at least some of this may come from dark matter.
Dark matter is a type of matter that is thought to account for apparent effects due to mass where no mass can be observed. It behaves differently to normal matter, such as planets and stars, which only accounts for approximately 5% of the universe. It neither emits nor absorbs light or other forms of electromagnetic energy, so a simple definition is that it is matter that does not react to light. The total mass-energy of the known universe is estimated to contain approximately 27% dark matter.
Using data collected from NASA’s Fermi Gamma-ray Space Telescope, scientists from different institutions generated maps of the center of the galaxy. They found that some of the high-energy gamma rays could not be sufficiently explained by known sources. There are numerous known sources of gamma-rays in the center of the galaxy, such as supernova remnants, but it is also predicted to be rich in dark matter. Although scientists know dark matter exists, they are not entirely sure of what it is composed of. Weakly Interacting Massive Particles, or WIMPs, are a strong candidate. It is thought that collision of WIMPs may produce a quickly decaying particle, which could produce gamma rays detectable by Fermi.
Once they removed all the known sources of gamma rays from the Fermi observations, some emission was leftover. If dark matter particles with a particular mass are destroying each other, this would be a remarkable fit for the remaining emission. Despite this, the scientists err on the side of caution since alternative sources may still exist. Further sightings are also required to make this interpretation more convincing.
The Fermi scientists have also turned elsewhere in an attempt to detect dark matter by looking at dwarf galaxies orbiting the Milky Way. Dwarf galaxies are rich in dark matter and lack other types of gamma-ray sources present in the center of the Milky Way which make detection of dark matter problematic. On the flip side, their distance from us and the fact that the dark matter present is still considerably less than that in the center of the Milky Way means that the signals are weak. But according to Elliott Bloom, a member of the Fermi collaboration, “If we ultimately see a significant signal, it could be a very strong confirmation of the dark matter signal claimed in the galactic center.”
While at this stage the signal cannot be confirmed or refuted as dark matter, it represents an exciting step towards the detection of dark matter at the galactic center.
Check out this YouTube video for an image of the Milky Way with the gamma-ray map from NASA's Fermi superimposed on top.
Credit: NASA Goddard; A. Mellinger, CMU; T. Linden, University of Chicago