Jie Shi

A supermassive black hole drifts through a cloud of dark matter. In the usual picture, this invisible substance just adds mass and barely interacts with ordinary matter. But dark matter particles could have intrinsic rotation — spin.

A black hole is not an empty funnel. Invisible matter can accumulate around it, and then the familiar boundaries shift. A group of theorists found a simple condition: if the mass of this surrounding material grows smoothly and satisfies the inequality 1 − 2m' − rm'' > 0, then the event horizon remains well-defined and the hole stays stable.