No it's not.
It's an interpetration of the apparent evidence.
Not as clear cut as saying the sun rose this morning, and we all saw it. That is a fact. But not science.
The big bang theory is relatively recent, and controversy and contradictions and revisions have been a constant companion to its "development." Contradictions and unexplained phenomena abound.
Even if there was a big bang, something had to exist to blow up. What was it that went bang? Nothing?