Galaxy
 Gravity holds galaxies together, against the general expansion of the universe. 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.