Astronomers say they may have seen the first potential signs of dark matter interacting with a force other than gravity, meaning it's not quite as "dark" — invisible except for its gravitational effects — as previously thought.

A group of astronomers has studied four galaxies simultaneously colliding with each other in a galaxy cluster known as Abell 3827.

They compared the theoretical distribution of dark matter against the mass where the system lies, as indicated by the position of the colliding quartet of galaxies.

What they discovered is that the clumps of dark matter in the system are apparently lagging behind the particular galaxy each is associated with.

Theories have suggested such a lagging — or offset — could occur during collisions of galaxies if dark matter clumps were interacting with each other, even slightly, through a force other than gravity.

Visible matter in each of the four galaxies is behaving as expected during the collisions, but the dark matter clouds surrounding each one appear to be dragging on each other, with the result that they are lagging behind the moving galaxies by at least 5,000 light years.

There has never been a previous observation of dark matter interacting with anything else or in any manner other than through gravity, researchers note.

The new finding potentially contradicts the standard theory of Cold Dark Matter, which posits that dark matter interacts only with gravity, they say.

"We used to think that dark matter just sits around, minding its own business, except for its gravitational pull," says lead researcher Richard Massey of Britain's Durham University. "But if it slowed down during this collision, this could be the first dynamical evidence that dark matter notices the world around it."

"Dark matter may not be completely 'dark' after all."

Dark matter is thought to make up about 85 percent of the universe's mass, and if it weren't for its gravitational effect on the galaxies it surrounds, those galaxies would fly apart as they rotate.

"We know that dark matter exists because of the way that it interacts gravitationally, helping to shape the universe, but we still know embarrassingly little about what dark matter actually is," says research team member Liliya Williams at the University of Minnesota.

"Our observation suggests that dark matter might interact with forces other than gravity, meaning we could rule out some key theories about what dark matter might be."

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