These two immense star systems are bound to each other by gravity. Together with a small number of other neighboring galaxies, they form part of a cluster. The universe is made up of an untold number of clusters of galaxies. Some clusters are bound by gravity to other clusters, forming superclusters. But from that scale onward, gravity loses its grip.
Scientists find that the superclusters are moving away from one another.
In other words, the universe is expanding. This amazing discovery suggests that there was a beginning when the universe was in a much smaller and denser state. The birth of the universe is often referred to as the big bang. Some scientists seriously question whether man will ever be able to find out how the universe was born. Others speculate about ways in which our universe could have come into existence without an intelligent cause.