The "big bang" space expansion is an observed fact.
I would not agree that the "big bang" is an observed fact.
The observed recession of the other objects in the universe is a "fact" so far as we can tell (although there are other attempts to explain the observed red shifts).
But, extrapolation of that observed expansion backward in time to a single point, (or whatever nearly a single point that each theorist chooses to start saying "I don't know before that,") is still
just a theory.
It is the most popular theory, but it necessarily involves
assumptions of additional, unverified matter and energy that we do not understand for
95% of its physics. Calling that a "fact" is closing ones mind to other possibilities. We should not be doing that - it is unscientific.
In order to advance our understanding, we need to keep track of what we can actually measure, and what we can only assume. Otherwise, we will be deluding ourselves as well as our listeners.
Now that we have a new, more powerful telescope that was designed to see what we think should be the "first" stars, and are discovering all sorts of unexpected things, we are being told that there are even more unexplained parameters that can be used to "fix' the Big Bang Theory.
Those are just more assumptions.
And, the very foundation of our cosmological models is
assumptions about things being the same (roughly) throughout the universe. If we were to allow the speed of light and the force of gravity to change with position and time, and the rate of time passage to change unevenly in different parts of the universe, or space to flow as well as expand and "bend", we could match pretty much any observations.
But, somehow, those alternate assumptions are "forbidden" by the same theorists who want to enshrine their own newly proposed time variations in "dark energy" and "dark matter" that make the new observations still not invalidate their favorite theory.
And, even with all of those assumptions, we still don't have a theory that explains why there is not as much anti-matter in the universe as there is regular matter.
The closest we get to "facts" is the Theory of Relativity, which does an excellent job of explaining and predicting what our observations are and will be at our
only tiny sample of the universe, where we are and when we are, now. It does not actually explain
why things appear that way. And, it is really only an assumption that they would appear the same from a much different location and/or at a much different time. It can't even
explain the observed speeds of stars in galaxies without adding the wildly tunable assumption of undetectable "dark matter" in whatever amounts are need in whatever locations are needed.
So, please stop pushing assumptions as facts.