Question: Are black holes rare to find in space, or are they pretty common? (I hope you win the vote!)

Sam Connolly answered on 17 Jun 2014:

Thanks, I hope I win too! The others are good too though, it’ll be close!

Black holes are pretty common, but nowhere near as common as normal stars. There are billions of galaxies we can see in space, and we think that every one of them has a supermassive black hole in the middle (which is a million to a billion times as heavy as the sun!) and hundreds or even thousands of smaller ones (which are a few times heavier than the sun). We know for certain that at least about a quarter of galaxies have supermassive ones, because they’re sucking up gas at the moment, which means we can see them. And we can see hundreds of smaller ones in our own galaxy, the milky way, and think there are more than we can see. Plus we can see a few smaller ones in nearby galaxies. So there are lots, billions! But even though there are loads, there are a lot more stars, because there are billions of stars in every galaxy, as opposed to only hundreds or thousands of black holes! So compared to stars, they aren’t so common.