Why are galaxies not expanding, but the entire universe is?
It’s the space between the galaxies that’s expanding. Galaxies are held together by gravity. They are like the raisins in the cake: as the cake bakes, the raisins don’t expand, they just get farther apart. If you have two raisins stuck together in the cake, they won’t move apart as the cake expands, either. In the same way, two objects gravitationally bound together don’t move apart as space expands.
