The keyword in ypur response is test. Through observations and repeatable experiments. You can't do that with something that allegedly happened billions of years ago like the big bang
Discussion
We can and absolutely do.
You can't and absolutely do not, at best you make estimated guesses which can never be conclusively proven because nobody observed the big bang and nobody has successfully repeated the creation of a universe through a "big bang". It is a pseudoscience and quasi religious because you blindly believe it with out any proof of its truth.