Galaxies are moving apart as the space between them continues to expand.

The longer version

Cosmic expansion stretches the space between galaxies rather than flinging them through it. Light traveling across that stretching space is shifted toward longer wavelengths, so distant galaxies appear redder than nearby ones, and the effect grows with distance. Every distant region seems to recede from every other.

This observation underpins the idea that the universe was once much denser and hotter. Galaxy groups bound by gravity resist the expansion locally, which is why the Milky Way and its neighbors stay together. The rate of expansion is measured through careful distance and redshift surveys. Distant objects were therefore closer together in the past.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.