I. Heywood

A black hole is an insatiable cook, and the star beside it is a pot of boiling soup. The cook ladles out the soup, but some spills: similarly, GX 339-4 pulls in gas from its neighboring star, but it doesn't hold onto all of it. The excess matter shoots out from the black hole's poles in two narrow jets, like splashes from an overfilled pot.

A galaxy is like a giant merry-go-round: stars and gas whirl around the center, and their speeds reveal how much stuff is on the ride. If we only saw the luminous matter, the outer parts would spin slower than observations show. So there must be invisible mass, dark matter, that holds the galaxy together.