The Big Bang theory is currently the best theory we have to explain the origin of the universe. Modern science believed that the universe had always been around. Einstein had developed his theory of relativity which actually predicts a start to the universe. He thought this can't be right and so added a fudge factor (Einstein's Cosmological Constant) to maintain what many scientists held to be true that the universe had always existed. A physicist by the name of George Lemaître looked at the equations and realised that the fudge factor should not be there. He wrote a paper explaining his theory that the universe was expanding having started from some 'primeval atom'. Einstein said he was wrong. When Hubble discovered evidence that the universe was expanding and some scientists were getting together to work out how to understand this, George sent his paper to them. They realised he got it right and Einstein then congratulated George on his achievement and they travelled together giving a lecture series on their understanding. You can read about it in, "'A Day Without Yesterday': Georges Lemaitre & the Big Bang".
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