the Universe all of space and all the matter and energy in it.  EVERYTHING!!!
Big Bang Theory theory that explains how the universe began, states that the universe began as a single clump of everything (a singularity) and that it EXPLODED and has been expanding ever since.
Big Bang happened about 14 billion years ago.
Red Shift light from an object moving away from you has a red shift to it. All galaxies and stars have a red shift when observed through a spectroscope.
Edwin Hubble first to use the Red Shift to support the Big Bang, showed that universe was HUGE, that other galaxies existed outside the Milky Way.
galaxies contain billions upon billions of stars.