Their detailed properties provide numerous tests of Albert Einstein’s general theory of relativity, including some of the strongest evidence to date for the existence of black holes. Most of the events are mergers of two black holes. Those include the largest black-hole merger yet and the most ‘lopsided’ one—in which two black holes of vastly different masses collided. Astrophysicists expected a sharp cut-off, with no black holes weighing more than 45 times as much as the Sun. It shows that, in some binary systems, the two black holes have misaligned axes of rotation, which would imply that they formed separately.