Question: do all spiral galaxies have a black hole in their centre?

Sam Geen answered on 17 Jun 2013:

Pretty much, yep! If you measure how fast galaxies are rotating, they seem to have a big mass at the centre that we can’t see. We can also see stars in our own galaxy orbiting around seemingly nothing – i.e. there’s a big invisible thing with a lot of gravity there. Scientists are still unsure why they exist, and it’s still debated a lot by people who study them.