The World According to Physics

by Jim Al-Khalili

Remember also that this expansion only happens in the vast expanses in between the galaxies, because within the galaxies themselves the gravitational field that holds them together is strong enough to withstand the overall cosmic expansion. Galaxies are like the raisins embedded within a loaf of rising bread in the oven. The loaf expands, but the raisins themselves remain the same size—they just become more separated from each other.