Dark matter is dark, but not totally dark

A few weeks ago, we reported that dark matter — the mysterious, quite unknowable stuff that makes up a large portion of the mass of the universe — was even darker than previously thought. Now the same researchers involved in that study report that dark matter may be super-duper dark, but not totally dark. Their new findings were published in the Monthly Notices of the Royal Astronomical Society.

The “darkness,” in this case, refers to the matter’s ability to not interact with itself, or anything at all but gravity: When clusters of galaxies collide, it seems that the dark matter inside them can butt up against other dark matter (and gas and dust) without slowing down. If you’ve ever collided with another human (or a wall), you know this isn’t how “normal” matter works.

In the new study, the researchers turned their gaze from galaxy clusters to four individual galaxies colliding simultaneously. When they studied collisions on that scale, they saw that clumps of dark matter lagged behind their galaxies in the aftermath — something that they believe to be the result of tiny bits of friction created during the collision.

Lead author Richard Massey of Durham University’s Institute for Computational Cosmology explained that the two studies don’t contradict each other.

“In our previous work we said, wow, dark matter interacts very little with anything,” he said. “In this new study we’re saying, OK, so it’s less than point five, but not zero.”

In between nothing and very little, he said, is the mystery of dark matter just waiting to be solved. And now he and his colleagues can continue to home in on its behavior.

“We looked at a galaxy that’s moving through a big soup of dark matter, and sure enough we’ve seen that dark matter has ended up — for whatever reason — in a different place.”

Five thousand light years away from its galaxy, in fact. That suggests that the dark matter interacted with something other than gravity to steer it off course.