Since I don ' t actually understand how they came to the conclusion of the Big Bang theory being the most accurate modelI ' m going to answer that instead of if it is a belief or not to accept it without understanding . Hubble the guy , not the telescope observed that the more distant a galaxy is , the more red it appears . That ' s something we understand well and it works like the Doppler effect . You know when a car is coming your direction it sounds sharp and when it passes you it becomes more grave . Light works the same way and if something is going towards you it appears more blue , if it ' s going away it looks more red . By measuring exactly how red the distant galaxies were , Hubble deduced their speed . So he figured out that all the objects in the universe are going away from each other , hence they used to be close together . The further back in time , the closer together they were until a moment when they were all on top of each other . Through the equation of relativity it was possible to figure out that if that did happen , there should have been a moment , about 300 000 years after the big bang , when the universe became transparent , before that it would have been opaque . So if we look 13 . 7 billion light years away we should see the universe being surrounded by something like a black wall with a temperature of about 270 C . We ' ve looked , and we ' ve seen exactly that wall . We call it the cosmic microwave background .