Why do galaxies move away from each other as the universe expands?

On large scales, the universe is expanding because the metric of space itself is increasing with time. That means most distant galaxies recede from us not because they are flying through a static void, but because the space between galaxies is growing. Edwin Hubble showed in 1929 that farther galaxies have larger redshifts, which became evidence for expansion. The rate of expansion is described by the Hubble constant, though its exact value is still debated between different methods. Over very large distances, expansion can even make some galaxies recede faster than light without violating relativity, because the motion is due to expanding space.