Dark matter detected in cosmic crash

Astronomers say fresh imagery of a powerful collision of galaxy clusters supports the idea that dark matter is something totally separate from ordinary matter.

The researchers used NASA’s Chandra X-ray Observatory and Hubble Space Telescope to study the galaxy cluster, known as MACSJ0025.4-1222.

They could see a clear separation between ordinary matter and dark matter, a mysterious constituent of the universe that can be detected only by its gravitational effect.

The research, which is due to appear in a future issue of The Astrophysical Journal, appears to answer a crucial question about whether dark matter interacts with itself other than via gravitational forces.

"It is in our view an important step forward to understanding the properties of the mysterious dark matter," Marusa Bradac, a physicist at the University of California at Santa Barbara, said Wednesday in a news release.

"Dark matter makes up five times more matter in the universe than ordinary matter," Bradac explained. "This study confirms that we are dealing with a very different kind of matter, unlike anything that we are made of. And we're able to study it in a very powerful collision of two clusters of galaxies."

The findings follow up on similar observations of the Bullet Cluster in 2006. Like that cluster, MACSJ0025 formed after an energetic collision between two large galaxy clusters. MACSJ0025 is farther away from Earth (5.7 billion light-years vs. 3.4 billion light-years), and it does not have the bullet-shaped cloud of gas seen in the Bullet Cluster.

Using optical images from Hubble, the team was able to trace the distribution of MACSJ0025's total mass — including dark as well as ordinary matter — thanks to a technique known as gravitational lensing. This method uses the distortion that mass causes as light passes by another object between the viewer and whatever is being observed.

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The Chandra X-ray images helped astronomers map the distribution of the ordinary matter, mostly in the form of hot gas.

As the two clusters collided and merged at speeds of millions of miles per hour, the hot gas from each cluster collided and slowed down, but the dark matter did not. That separation provides evidence to support the view that dark-matter particles interact with each other only very weakly or not at all, apart from the pull of gravity.

Dark matter is thought to make up 23 percent of the universe's mass-energy content, compared with only 4 percent for ordinary matter. Dark energy, another mysterious constituent of the universe thought to be responsible for its accelerating expansion, accounts for 73 percent of the cosmos, physicists say.

Some scientists theorize that dark matter is composed of exotic subatomic stuff known as weakly interacting massive particles, or WIMPs.

Others say dark matter includes black holes and brown dwarfs that radiate little or no light — things that are known collectively as massive compact halo objects, or MACHOs.

This report includes information from Reuters and msnbc.com.

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