The giant holes in galaxies’ centers shouldn’t be able to merge, yet merge they do. Scientists suggest that an unusual form of dark matter may be the solution. Introduction Galaxies have been merging into ever-bigger structures over the course of cosmic history. When galaxies merge, the supermassive black holes that sit in their centers must eventually merge, too, forming an even more gargantuan black hole. For decades, though, a question has vexed astrophysicists: How can supermassive black hol...