The objects which are the "building blocks" of the universe, and are observed by astronomers to move away from us are called galaxies. A galaxy is a huge collection of stars (about 100 billion stars), with each galaxy appearing to be an "island" in the vastness of space. Our own Sun is part of a galaxy called the Milky Way.

Scientific evidence that the universe is expanding requires (a) proof that galaxies are moving away from our galaxy (the Milky Way), and (b) proof that the distance between our galaxy and other galaxies is related to their speed away from us.

How far has the universe expanded since its beginning? Sizes of the structures formed give clues to the answer. Nearest stars: 10 light years Milky Way: 10^5 light years Local Group: 4x10^6 light years Galaxy Cluster (Virgo, Coma): 25x10^6 light years Local Supercluster: 150x10^6 light years Universe: 15x10^9 light years