And, all the galaxies we have observed are moving away from us, and each other, which implies that they were once closer together. The current ratio of hydrogen to helium, the two simplest atoms, is exactly what is predicted by the Big Bang concept. These are just several of many, many pieces of evidence that are explained by the Big Bang theory. The theory of evolution is similar - it is an attempt to explain the physical world based on the evidence we have. Scientific theories are not absolute, nor are they complete.
Theories are always tentative to a certain degree.
One never knows what new thing might discovered tomorrow. Theories change when they conflict with facts. Scientific theories are constantly being extended and improved to explain more facts. Sometimes this takes a lot of time. Anomalous facts are often treated very skeptically, particularly if people have a lot of confidence in the theory.