for National Geographic News
New observations of a massive intergalactic collision are raising more questions about the mysterious nature of dark matter.
The study looked at three huge galaxy clusters that are merging into an even bigger cluster called Abell 520, located about three billion light-years from Earth.
Astronomers describe the collision as a "cosmic train wreck," given the immensity of the forces involved. Each cluster contains about a thousand galaxies, and each galaxy has billions of stars.
The new findings show an unprecedented mix-up in the merging clusters, suggesting the need for an "uncomfortable" revision—or entire rewrite—of our current theories of dark matter.
"Whatever happened did something really unusual to the galaxies," said study lead author Andisheh Mahdavi, an astronomer at the University of Victoria in British Columbia, Canada.
"It moved them all the way on the outer edge of the central region of the cluster, so that only gas and dark matter is left at the center," he added.
"That's never been observed before, and it's really hard to explain."
The findings are slated to appear in an upcoming issue of the Astrophysical Journal.
Dark matter does not absorb or emit light, but scientists believe it makes up about 90 percent of the matter in the universe.
Prevailing theories hold that dark matter is composed of particles that have very weak interactions and move only under the influence of gravity, just like stars.
So when galaxies collide, scientists expect stars and dark matter to move together. Intergalactic gas, however, also responds to pressure and thus is expected to lag behind the other matter.
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