For a long time men wondered how the universe would have emerged. Gradually it was necessary to abandon the idea that we occupy a central position in the universe and adopt the view that our location in the universe is insignificant.
The Big Bang Theory assumes that galaxies are moving away from each other, as noted by Edwin Hubble in 1930. Thus, it is assumed that in the distant past, some 10 to 15 billion years ago, all galaxies were at one point at a very high temperature that expanded into the Big Bang.
So although the name "big Bang"brings us to the idea of a kind of explosion, in fact, what happened was an expansion from a tiny (and very dense) state to what it is today. In other words, the Big Bang Theory does not have the purpose of explaining what initiated the creation of the universe, what existed before the Big Bang or even what exists outside the universe, and, yes, how it "transformed" into what we now call the universe.
Belgian priest, civil engineer and cosmologist Georges-Henri Lemaître was most likely the first to propose a model for the Big Bang in 1927. He imagined that all matter was concentrated at one point, which he called primordial atom, and that this atom had broken into many pieces, which were fragmenting more and more until they reached the atoms we know today. The hypothesis raised by Lemaître is the first idea that there would have been a nuclear fission (process in which a heavy atom splits into lighter, more stable nuclei).
Although incorrect, since Lemaître's hypothesis violates the laws of the structure of matter, it inspired modern models of theories about the origin of the universe.
Regardless of Lemaître, Russian mathematician and weatherman Alexander Friedmann discovered a whole family of solutions to the equations of General Relativity Theory (This is the theory of gravity, describing gravitation as the action of the masses on the properties of space and time, which ends up not only affecting the motion of bodies, but also other physical properties).