system
Given the following post, respond as if you're commenting on Reddit:
human
The ten black hole merger events detected by LIGO over the past five years seems like a lot. Is it?

LIGO has been running for five years, or about one three billionth the age of our universe.   I'm amazed that it has already detected ten black hole mergers.   Can we interpret this as: 1) there are a LOT of black holes in the universe.   2) the universe is SO huge and mind-boggingly expansive that even rare events are common. 
 3) there are black hole clusters.  or 4) we are seeing mergers from a time when black holes were more common.
gpt
Space is really big and has a lot of stars. If the average distance between galaxies is 3 million lightyears, there are 100 billion stars per galaxy, and LIGO has a detection range of 1 billion lightyears, that makes something like 10^18 stars in the detection range. A tiny fraction of those are big enough to form black holes, and a small fraction of those are in binary orbits. But those that are in binary orbits eventually collide.