NASA's Fermi Gamma-ray Space Telescope may provide the best evidence yet for the existence of dark matter. The evidence comes from gamma-ray seen streaming away from the center of our own galaxy.
Dark matter is a mysterious substance which makes up the vast majority of the matter in the Universe. It cannot easily be detected, providing the name, but gravitational effects on stars and galaxies suggest its presence.
Fermilab researchers worked with colleagues from the Harvard-Smithsonian Center for Astrophysics (CfA), the University of Chicago and the Massachusetts Institute of Technology (MIT) to study data collected by the orbiting observatory. They found the galactic center is producing a far greater amount of gamma rays then current theories predict. This high-energy electromagnetic radiation is consistent with forms which could be produced by certain forms of dark energy, the researchers reported.
"The new maps allow us to analyze the excess and test whether more conventional explanations, such as the presence of undiscovered pulsars or cosmic-ray collisions on gas clouds, can account for it. The signal we find cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models," Dan Hooper, an astrophysicist from Fermilab, and a lead author of the study, said.
Dark matter is believed to draw matter together through gravitation, assisting the buildup of galaxies. Astrophysicists are still uncertain as to the exact nature or composition of dark matter, as it has never been directly measured. Gravitational effects on normal visible matter are the only evidence dark matter provides to reveal its existence.
WIMP's, or Weakly Interacting Massive Particles, are a leading candidate to comprise dark matter. These theoretical sub-atomic particles only rarely interact with matter, making them "dark." Theories predict these particles will annihilate, releasing gamma radiation. Radiation from these annihilations will happen at very specific energies, according to the theories.
The galactic core of the Milky Way is teeming with gamma-ray sources, including binary star pairs and supernova remnants and pulsars. Once the researchers determined the amount of energy coming from the galactic center, they subtracted radiation from all known sources. Once they did this, they found evidence that could be linked to the mysterious substance. The gamma-ray excess was found to have the energy levels predicted by the theories.
"[We found] the GeV excess to be robust and highly statistically significant, with a spectrum, angular distribution, and overall normalization that is in good agreement with that predicted by simple annihilating dark matter models," researchers wrote in the study announcing the results.
Current models of the universe suggest it is composed of 26.8 percent dark matter, while less than five percent is ordinary matter. The rest is dark energy.