The Big Bang Theory is the prevailing theory of the origin of the universe, and it is based on astronomical observations. According to this theory, about 15 billion years ago all the matter and energy in the visible universe was concentrated in a small, hot, dense region, which flew apart in a gigantic explosion.

Before the twentieth century, most scientists believed the universe was static in the sense that it was neither growing nor shrinking as a whole, although individual stars and planets were moving. In 1915 Albert Einstein proposed the general theory of relativity, which is a theory of gravity that has superseded Isaac Newton's theory of gravity for very massive objects. Since general relativity was invented, its equations have been used to describe the possible ways in which the universe might change as time goes on. Einstein, like others before him, thought the universe...