Big Bang Theory is one of the most accepted model to explain the evolution and expansion of Universe from its origin at singularity point.There exists a lot of evidences to prove that our universe had a beginning and since then has been expanding. Big Bang Theory is based on some observation of perceivable universe and mathematical theories like general theory relativity and quantum and standard theories of particles.
Before Big Bang there was a Singularity, everything that exist today was inside it but in a form that we can’t explain neither we can explain how and why this expansion started and what was the cause for this expansion to happen. All that we know is what happen after this expansion.
First in 1927, Georges Lemaître, an astronomer said that universe had a beginning and initially it was very small then it expanded to be as big as it has become today. Then in 1929 Edwin Hubble discovered that galaxies are moving away from each other and ones that are far away from us are moving much faster than the closer ones.