Since the breakthrough in gravitational wave astronomy back in 2015, scientists have been able to detect more than a dozen pairs of closely located black holes—known as binary black holes—by their collisions into each other due to gravity. However, scientists still debate how many of these black holes are born from stars, and how they are able to get close enough for a collision within the lifetime of our universe.
Click here for original story, How many stars eventually collide as black holes? The universe has a budget for that
Source: Phys.org