Before the twentieth century, astronomers could only assume that the universe had existed forever without change, or that it was created in its present condition by divine action at some arbitrary time. Evidence that the universe may be evolving did not begin to accumulate until the 1920s. Today, the idea that all matter in the universe was created from a gigantic explosion called the "big bang " is widely accepted by students of cosmology.

It was Albert Einstein's theory of relativity, published in 1915, that set the stage for the conceptual development of an expanding universe. Einstein had designed his theory to fit a static universe of constant dimensions. In 1919 a Dutch astronomer, Willem de Sitter, showed Einstein's theory could also describe an expanding universe. Mathematically, de Sitter's solution for Einstein's equation was sound, but observational evidence of expansion was lacking, and Einstein was skeptical.

Beginning...