Dark matter isn’t interacting with itself after all

Hints that a galactic collision knocked the invisible mass askew are disproven.

Dark matter is still the shyest particle in physics. New observations show that dark matter in galaxy cluster Abell 3827 stubbornly ignores all other kinds of matter — including itself, astronomers reported April 6 at the European Week of Astronomy and Space Science in Liverpool, England.

The research, also posted online at arXiv.org, negates an earlier finding that stars were separated from their dark matter in Abell 3827, a cluster including four colliding galaxies about 1.3 billion light-years from Earth (SN: 5/16/15, p. 10).

At the time, cosmologist Richard Massey and colleagues suggested the dark matter may have lagged behind its galaxy because it was interacting with another clump of dark matter — something dark matter is not supposed to do, according to standard theory. Dark matter, which makes up most of the mass of the universe, is only known to interact with ordinary, visible matter via gravity.

But more recent observations made with the Atacama Large Millimeter Array in Chile show that the dark matter was actually behaving exactly as expected.

“We looked for longer and found the dark matter was hiding just where it ought to be,” says Massey, of Durham University in England. “It’s a sort of eating humble pie on some level.”

It’s still possible that other galaxy clusters will reveal lagging clouds of dark matter, Massey says. His team has designed a balloon-borne telescope called SuperBIT, which they hope to use to check hundreds of galaxy clusters for misbehaving dark matter.

“We just know embarrassingly little about it,” says Massey. “We keep trying to take a step forward, and find ourselves going back to the beginning.”