The discovery that the universe is expanding also told astronomers something about how the universe might have formed. Before this discovery, there were many ideas about the universe, most of them thinking of the universe as constant. Once scientists learned that the universe is expanding, the next logical thought is that at one time it had to have been smaller.

Source

David Bethel http://ck12.org/flexr/assemble/?fid=732 (CC BY-SA)