The Big Bang theory is the result of about 80-90 years of scientific data pointing to the fact that the universe, from what we can observe and mathematically model, seems to have one original starting point. By "universe," it is generally held that this description entails all observable matter and energy. Later models demonstrate that the forces like gravity and electromagnetism, as well as space-time also share their origin with what is believe to have been a near-infinite, hot and dense quantum singularity surrounded by absolutely nothing, much like a black hole. We came to this conclusion by observing the way in which the universe moves and is shaped, and accounting for what could cause such a pattern. Big Bang theorists have also dedicated a great deal of time and resources eliminating other possibilities which may have accounted for this pattern, making Big Bang theory a very strong contender for an explanation of how the universe evolved. It is in first place with no real competition from other theories.
It should be noted that Big Bang theory has nothing to say about what came before the universe which remains unknown. It also has little to do directly with biological evolution. The chief opposition to Big Bang theory appears to be religion, particularly creationism.