sentence: Gravity holds galaxies together, against the general expansion of the universe.
sentences_after: In effect, the expansion of the universe takes place between groups of galaxies, not inside those groups. This is because the mass within a galaxy holds the galaxy together, and the same applies to the groups of galaxies, such as our Local Group. The gravitation is produced by the matter and energy in a galaxy or group of galaxies. Everything in a galaxy moves around a centre, and that is also an effect of gravity.
concept_name: gravity
headings: Galaxy