What force holds the stars in a galaxy together?
Fusion energy keeps each star together as a star. Do you mean what keeps a bunch of stars together to form a galaxy? I’m not sure anyone really knows, unless it’s gravity. There is supposedly a giant black hole in the center of our galaxy, and scientists aren’t sure if that’s unusual, or if every galaxy might have a black hole in the center. Supposedly the mass of the galaxy should be enough to generate the gravity to keep it together. The problem is, the estimation of the mass, based on the number of stars, is not enough mass to do this, based on our current formula of gravity. So either the formula is wrong, or there is invisible “dark matter” that constitutes the missing mass.
