Some black holes at the centers of galaxies have a buddy, but detecting these binary pairs isn't easy

Every galaxy has a supermassive black hole at its center, much like every egg has a yolk. But sometimes, hens lay eggs with two yolks. In a similar way, astrophysicists like us who study supermassive black holes expect to find binary systems--two supermassive black holes orbiting each other--at the hearts of some galaxies.