The Big Bang theory of the Universe dates back to 1927, when the Belgian physicist Georges Lamaître proposed that an expanding Universe could be traced back to a single point where the Universe would have been in a very hot and dense state.
Evidence for the Big Bang theory came in 1932 when the American Astronomer Edwin Hubble observed that the distance to far-away galaxies was proportional to their redshift: the shift in the wavelength of the light they emit because they are moving away from us.
The redshift of the light is due to the Doppler effect. The Doppler effect is responsible for police sirens changing pitch when the police car passes by. As the car moves towards you the wavelength of the sound is compressed but as it starts moving away the wavelength of the sound is stretched out. If galaxies are moving away from us now, then at some time they must have been closer together.
In the modern picture of the formation of the Universe, the Universe starts of in this hot dense state and rapidly expands and cools to form the building blocks of nature. The first protons and neutrons start to appear a few seconds after the “Big Bang”. At the time of the Big Bang there should have been equal amounts of matter and antimatter. Where is all the antimatter today?
In 1967 a Russian physicist named Andrei Sakharov proposed three conditions for generating a matter dominated Universe: CP must be violated such that reactions involving matter particles and antimatter particles can happen at different rates; there must be a mechanism to produce more baryons than antibaryons; and the interactions must happen out of thermal equilibrium otherwise the asymmetry would be undone as fast as it is created.