When galaxies collide, it’s not a gentle affair, but it does take millions of years. Over this time, the two massive star systems slowly merge together, their gravitational pull drawing them closer. At the heart of each galaxy lies a supermassive black hole, an object containing millions or even billions of times the mass of our sun. After the galaxies merge, these two black holes should eventually find each other, settling into orbit around their shared center of gravity. The result is one of the universe’s most extreme phenomena, a supermassive black hole binary. But to date, none have been found.