How much percent are we certain from 1 to 100% that we are sure the universe expanded from a big bang? We know that the universe is full of dark matter the possible driving force of the universe, the universe would have to have started from a finite dense point and expanded into what we see it today.
An infinitely dense point (singularity) is a mathematical concept. The reality was a very small hot dense state as a starting point. Evidence for the BB is the prediction of the temperature of the CMBR. The error bars on the prediction were set a 2-5K, with the “best guess” placed at 3K (if I recall correctly). The measured temp was 2.7K – so they knew what they were talking about there. It was predicted that the CMBR would be highly isotropic. Turned out that that was also true. And finally, the CMBR must show all the other characteristics of black-body radiation. And there is a “photograph” of the BB – WMAP (much larger than the beginning but that’s when it became visible). The Boomerang and COBE missions were the first instruments to confirm these predictions. You can research more on you own about the additional information developed to confirm the BB.