Scientists don’t know for 100% that the big bang did happen. When they study how the universe is today and how’s it’s changing they developed a model to describe it. This model is called the Standard Model and starts with a Big Bang.
This model can explain everything we can see in the Universe and all the rules of Physics behave as the model predicts. Therefore it seems very likely that the Big Bang did happen, but it is possible that some new discoveries may not behave as we predict and then we might have to rethink the Big Bang.