Astronomers have perhaps their best lead to date about the nature of dark matter, the strange and invisible stuff that dominates the material universe.
The center of our Milky Way galaxy generates more high-energy gamma rays than can be explained by conventional sources such as supernova remnants and fast-spinning, super-dense neutron stars known as pulsars, a new study suggests. The "excess" may be produced by the annihilation of colliding dark matter particles. This NASA video explains the tantalizing dark matter find.
"This is a very exciting signal, and while the case is not yet closed, in the future we might well look back and say this was where we saw dark matter annihilation for the first time," co-author Tracy Slatyer, a theoretical physicist at the Massachusetts Institute of Technology in Cambridge, said in a statement. [The Hunt for Dark Matter: Images and Photos]
Dark matter— which is thought to make up more than 80 percent of the matter in the universe — is so named because it apparently neither absorbs nor emits light, making it impossible to observe directly with telescopes. But its gravity does affect the "normal" stuff we can see and touch, providing one way to hunt dark matter down.
Gamma rays — the most energetic light in the universe — also provide another potential detection method.
Many scientists think dark matter is primarily composed of Weakly Interacting Massive Particles, or WIMPs. Theory suggests that some types of WIMPs annihilate when they collide with each other, while others generate a fast-decaying secondary particle when they interact. In either case, the idea goes, gamma rays are produced.
In the new study, researchers used data from NASA's Fermi Gamma-ray Space Telescope to make maps of the Milky Way's center in gamma-ray light. The maps reveal an "excess" of gamma-ray emissions extending outward at least 5,000 light-years from the Milky Way's core, researchers said.
"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," said lead author Dan Hooper, an astrophysicist at Fermilab in Batavia, Ill. "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."
The excess can be explained by annihilations of dark matter particles with a mass between 31 and 40 billion electron volts, researchers said.
"Dark matter in this mass range can be probed by direct detection and by the Large Hadron Collider (LHC), so if this is dark matter, we're already learning about its interactions from the lack of detection so far," Slatyer said.
The study team is not claiming to have found a smoking gun for dark matter; additional data from other observing projects, the LHC and/or direct-detection experiments would be required to validate their interpretation, they stress.
"Our case is very much a process-of-elimination argument. We made a list, scratched off things that didn't work and ended up with dark matter," said co-author Douglas Finkbeiner of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge, Mass.
The new study has been submitted to the journal Physical Review D.