Mysterious Dark Matter Slides Through Colliding Galaxies Unscathed

Astronomers have taken a closer look at how dark matter in clusters of galaxies behaves when the clusters collide. They've found that dark matter interacts with itself even less than previously thought, which narrows down what the substance might actually be.

Dark matter makes up more of the universe than visible matter, but it's extremely elusive; it doesn't reflect, absorb or emit light. Because of this, it's only known to exist due to observations of its gravitational effects on visible matter.

In order to learn a bit more about dark matter, the researchers looked at galaxy clusters, where collisions involving dark matter happen naturally and where it exists in vast enough quantities to see the effects of the collisions.

Galaxies are made up of stars, clouds of gas and dark matter. During collisions, the clouds of gas spread throughout the galaxies crash into each other and slow down or stop. The stars, in contrast, do not have a slowing effect on each other.

"We know how gas and stars react to these cosmic crashes and where they emerge from the wreckage," said David Harvey, lead author of the new study, in a news release. "Comparing how dark matter behaves can help us to narrow down what it actually is."

After studying the collisions, the researchers found that, like the stars, dark matter continued straight through the violent collisions without slowing down. This is due to the fact that dark matter is spread evenly throughout the galaxy clusters, so dark matter particles frequently get very close to each other. The reason the dark matter doesn't slow down is because it does not interact with visible particles, and also interacts less with other dark matter than previously thought.

By finding out dark matter interacts with itself less than previously thought, scientists have successfully narrowed down the properties of dark matter. Now, scientists have a smaller set of unknowns to work with.

The findings are published in the journal Science.

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