Milky Way's Dark Matter Clumpier Than Thought
The universe’s normal and invisible dark matter is revealed in this portrait assembled from space telescope observations. Normal matter appears in red, its distribution observed primarily by the European Space Agency’s XMM/Newton telescope. The blue regions distinguish areas of invisible and elusive dark matter as recorded by the Hubble Space Telescope. The gray areas denote stars and galaxies, the visible light of which was also observed by Hubble.
Credit: NASA, ESA and R. Massey (California Institute of Technology)

Our galaxy's dark matter is clumpier than once thought, according to a new computer simulation.

The model, created by one of the most powerful supercomputers in the world, shows that the spherical halo of dark matter that envelopes the Milky Way contains dense clumps and streams of the mysterious stuff, even in the neighborhood of our solar system.

"In previous simulations, this region came out smooth, but now we have enough detail to see clumps of dark matter," said researcher Piero Madau, an astrophysicist at the University of California, Santa Cruz.

Dark matter, which scientists can only detect by noting its gravitational effect, is thought to make up about 85 percent of the matter in the universe. Its composition remains a mystery, though some scientists think it's made up of hypothetical particles called WIMPs (weakly interacting massive particles), which could annihilate each other and emit gamma rays when they collide.

The new simulation, described in the Aug. 7 issue of the journal Nature, implies that dark matter could be detected by the recently launched Gamma-ray Large Area Space Telescope (GLAST).

"That's what makes this exciting," Madau said. "Some of those clumps are so dense they will emit a lot of gamma rays if there is dark matter annihilation, and it might easily be detected by GLAST."

So far, though many teams have been looking for WIMP particles, no one has conclusively detected them.

"There are several candidate particles for cold dark matter, and our predictions for GLAST depend on the assumed particle type and its properties," said Juerg Diemand, a postdoctoral fellow at UCSC who led the new research. "For typical WIMPs, anywhere from a handful to a few dozen clear signals should stand out from the gamma-ray background after two years of observations. That would be a big discovery for GLAST."

The model took about one month to run on the Jaguar supercomputer at Oak Ridge National Laboratory in Tennessee. By following the gravitational interactions of more than a billion parcels of dark matter over 13.7 billion years, the computer could predict how the dark matter in the universe developed over time based on leading theories of how dark matter interacts.

"It simulates the dark matter distribution from near the time of the Big Bang until the present epoch, so practically the entire age of the universe, and focuses on resolving the halo around a galaxy like the Milky Way," Diemand said.

The research was funded by the U.S. Department of Energy, NASA and the Swiss National Science Foundation.