A galaxy is a cluster of stars. While a small galaxy may contain a few million stars, the larger ones contain a trillion stars or more. Our own galaxy, the Milky Way, contains several hundred billion stars. And there are about a trillion galaxies in our universe.
It has recently been found that the space between galaxies is occupied by rogue stars, stars that have been flung out of a galaxy. It has also been hypothesized that as many as half of the stars may actually reside in the region between galaxies.
Earlier it was thought that the space between the galaxies is filled with gases (probably remnants of galaxy and star formations) with a very low density (less than 1 hydrogen atom per cubic meter). This new discovery adds rogue stars to the mix.