The Big Bang was the start of our Universe. Early on in the big bang there was a massive concentration of stuff that wasn’t differentiated into matter and energy yet. Later on there was matter and energy. Later still there were Galaxies, stars and the rest of what we know in the universe.
The Big Bang is, at this point in science, not a matter of belief or disbelief or public debate. It is a scientific fact that the known universe underwent inflationary growth from a dense, hot, compact state 13- 14 Billion years ago (though the exact number is under constant refinement) which was likely the sole or primary source of origin for all macroscopic structures visible today. It says nothing about what "created" or "caused" the universe to exist, nor does it say anything about the origins of life or intelligence. It is factually known through detailed observation of the cosmic microwave background radiation which is analogous to an "echo" of the universe's expansion, as well as rigorous measurements of the distance and velocity of samples of the billions of galaxies observable in the universe.
The associated Big Bang Theory encompasses all such measurements, predictions, and extrapolations related to the historical fact of the Big Bang and accounts for a relatively clear understanding of the origins of what is visible today.
The Big Bang is a relevant topic of debate for many Creationists, however, as to them it represents a conflicting perspective to the Biblical origins of the universe described in Genesis. This is simply a false comparison, however, as the Big Bang itself is not about what ultimately caused the universe to come into existence, but about its observable history. Physical theories exist that offer naturalistic explanations for a first cause, but the ultimate verifiability of theories whose predictions are necessarily beyond observation within our own universe is a matter of ongoing scientific debate.
- Big Bang Theory Funnier