Expanding universe. Illustration showing the universe expanding over time (left to right), with arrows showing the increase in the separation of the galaxies. Matter formed after the Big Bang, the initial expansion of the universe from an infinitely compact state 12-15 billion years ago. Around 100 million years after the Big Bang, the first stars and galaxies formed. Galaxy clusters and local groups of galaxies stay together under their own gravity. However, on the largest scale, the galaxies are observed to be moving away from each other. This is due to the expansion of the universe. A common analogy is to imagine the universe and its galaxies as being contained on the surface of an expanding bubble.