<human>:  Who came up with the theory of the Big Bang?
<bot>: The big bang theory was first proposed by a man called Georges Lemaître. He proposed that the universe began with a sudden explosion, called the big bang. This was a major breakthrough in the understanding of the universe, because it showed that the universe is not static, but instead it is expanding. The big bang theory was the first theory that explained why the universe is expanding, and it led to the development of the Big Bang Theory.