Please stop saying two things that are fundamentally and inherently different things boil down to semantics.
Science works on levels of confidence - not absolutes. No one can say what definitely happened, only give you a level of confidence - based on evidence that a thing happened.
In this respect - science does not assert that any specific thing definitely happened - so the original question you asked is nonsensical.
In terms of speculation - almost all definition specify that speculation is conjecture that is not supported by firm evidence. Colloquially it’s used to mean spitballing or guessing solutions
Some scientific theories may very well have started off as the latter; and the progressed to the first once the idea was self consistent and could not be ruled out.
But those things are not “believed” or assigned a level of any confidence.
However, theories like the Big Bang - or the acceptance that there is some form of dark matter and dark energy in the cosmos is based on actual firm evidence, that confirms the claim. Thus is absolutely not speculation.
Perhaps you are confused at the difference between confidence and certainty - and are assuming that if one is merely confident then the reason for that confidence is speculation.
That’s just silly: if one has firm evidence that something is probably true; then suggesting it is probably true is not speculative any more. And in that respect - no accepted theory (explanations so well supported by data that we can claim that they’re almost certainly true) can be considered speculation.