Black holes on a collision course spin around one another with increasing speed as they draw closer. But very large black holes reach a point at about one parsec (3.26 light years) apart where their orbital velocity starts to balance out gravitational attraction. The decay of their orbits would happen so slowly that the actual merger could not happen within the current age of the Universe.
Nevertheless, physicists do believe such mergers happen, which requires new theories about how to overcome the so-called “final parsec problem”. Some additional force or energy is required to bring the orbiting black holes together.