A prediction from mathematical theory is not objective evidence.
But a mathematical theory based on a theory that has been verified by observation is. General relativity has been extensively tested. The BB is what you get when you apply GR to the universe as a whole.
Even if universal expansion is correct, not knowing the why and how of the BB leaves the theory questionable.
Why? It is a theory about how the universe is *after* the beginning.
You don't need a theory of planet formation to be able to describe the orbits of the planets.
For example, the universe may in fact be a multiverse and an essence already existed that became this expanding universe. This would make more scientific sense than a one off universe from essentially nothing without a clue as to why or how theory.
And that would have no effect on the BB description. If you understood what the BB model *actually* says, then you would know that.
That is all smoke and mirrors to avoid admitting that there is no objective evidence as to show how the BB began. By objective, I mean energy and mass from nothing evidence, not mathematical predictions.
Actually, we *do* have this, but within the universe. Quantum fluctuations do exactly this. And, in some models (extensions of the BB model), the BB is produced by exactly such quantum fluctuations.
But, I would point out, once again, that you are *assuming* that there was a 'before the BB'. And you do this without any good reason for that assumption.