In 1927, the German priest Georges Lemaitre was the first to propose what is known as the Big Bang theory, that the universe began with the explosion of a primitive atom. However, he called it his "hypothesis of the primeval atom." His proposal came after observing the change in the red distant nebulae astronomers to a model of the universe, on the basis of Einstein's Theory of Relativity as calculated by Alexander Friedmann. Years later, Edwin Hubble found experimental evidence to help justify Lemaître's theory. He found that distant galaxies in every direction are going away from us with speeds proportional to their distance.
The big bang was initially suggested because it explains why distant galaxies are traveling away from us at high speed. If the distance between clusters of galaxies are increasing today, all of them must have been closer in the past. The theory also predicts the existence of cosmic background radiation (the remaining glow of the explosion itself). The Big Bang Theory received its strongest confirmation when this radiation was discovered in 1964 by Arno Penzias and Robert Wilson, who later won the Nobel Prize for this discovery.
Sir Fred Hoyle FRS (24 June, 1915 – 20 August, 2001), English astronomer, is credited with coining the name "Big Bang" in 1949 during a radio broadcast. While meant as a derisory reference to a theory that he did not agree with, the name has become irretrievably connect with the theory.
After the discovery of cosmic background microwave in 1964, especially when their collective frequency curve traced a black body, most scientists are now quite convinced by the evidence that the Big Bang or a similar scenario had take place.
Although the Big Bang Theory is widely accepted, it probably will never be proven; consequentially, leaving a number of tough, unanswered questions.