Galaxy cluster smash-up lets us observe dark matter on its own

When two enormous clusters of galaxies collided billions of years ago, their dark matter shot right out of them, leaving behind the gas and stars that made up the remains of the clusters. Understanding this process could help us figure out the nature of dark matter and its effects on the universe.

Clashes between galaxy clusters are difficult to observe. We have to catch the collisions at exactly the right time, and at the right angle with respect to Earth, to…