A monster black hole [2] sits at the center of the Milky Way Galaxy. And it’s probably surrounded by many thousands of smaller black holes. But that’s just the beginning. There could be hundreds of millions of black holes spread throughout the galaxy.

A black hole is so heavy and dense that nothing can escape its gravitational pull — not even light. That means we can’t see a black hole on its own. But there are lots of indirect ways to see them, including their effects on the stars and gas around them. Astronomers have even taken a picture of one — a dark silhouette against a glowing background.

So far, a few dozen black holes have been confirmed in the Milky Way. The biggest is the one in the galaxy’s heart. It’s about four million times the mass of the Sun.

But most black holes are much smaller — from a few to a few dozen times the Sun’s mass. Such black holes form when massive stars collapse. Most of these black holes travel through the galaxy alone, so there’s not much way to see them.

The ones we know about have “normal” companion stars. Astronomers detect a black hole’s gravitational influence on such a companion, or they see it “stealing” some of the companion’s gas.

From these discoveries, and estimates of how many massive stars have lived and died in the Milky Way, astronomers deduce how many black holes the galaxy should host. The numbers vary. But most are in the range of a few hundred million — black holes galore in our home galaxy.

Script by Damond Benningfield