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The Big Bang theory is science's best explanation of how the universe was created. The theory asserts that our entire universe was created when a tiny (billions of times smaller than a proton), super-dense, super-hot mass exploded and began expanding very rapidly, eventually cooling and forming into the stars and galaxies with which we are familiar. This event is said to have happened approximately 15 billion years ago. Rather than expanding outward into some preexisting vacuum, the event of the Big Bang was space itself expanding - perhaps at speeds greater than light. (While Einstein's theory of relativity forbids anything within space from travelling faster than light, it sets no limitations on how fast the fabric of space itself may expand.)

The Big Bang theory was originally developed in the late 1920s by Georges-Henri Lemaître, a Belgian Catholic priest and astronomer, an early advocate of solutions to the general relativity field equations which predicted our universe was expanding. (For cosmological theories to be taken seriously, they must pose possible solutions to Einstein's general relativity field equations.) Though the expanding-universe solution to the field equations was derived by the Russian cosmologist Alexander Friedman in 1922, Lemaître was the first to realize that a continuously expanding universe implies that at some point in the past the universe must have been much denser and smaller, even atom-sized.