Science proceeds by steps:
--evidence or experiment suggests something
--hypothesis explains evidence and leads to better experiment or better avenues for research.
--More evidence fits with hypothesis
--Hypothesis becomes theory
--theory leads to collection of more evidence and better, more complete, explanation of evidence
At this point, most see that collection of more evidence will confirm theory.
If a lot of evidence accumulates that seems to invalidate the theory, the theory is adjusted to fit the evidence,
or thrown out in favor of a new theory that better explains the evidence.
FACT is not a scientific word. All theories are subject to revision to fit the evidence.
When dealing with "deep time", we simply haven't the minds to comprehend it. A universe billions of years old is beyond our understanding.
"Big Bang" is a bad name. A better term would have been Space-Time Origination, since a bang expands into something and, in this case, it was
the something that expanded from a point. Quantum fluctuations could explain the start, and perhaps trillions of years in the making.
We have no comprehension of that amount of time, and it's meaningless anyway, since there was no space and time prior to the Big Bang.
Some day we may be able to explain it with math, but it will be a hypothesis likely untested by evidence unless we can figure out how to observe
the starting point.
We make assumptions as a starting point for a hypothesis. If the evidence doesn't fit, out goes the assumption.
If an explanation started with an assumption and proceeded all the way to a working theory, that was a darned good assumption.