There are two responses to that (sorry, this is reiteration – but so is your assertion):
Evolution (common descent with modification, over eons) is a
fact. Natural selection is the
theory that explains the
fact. (Compare gravity. It’s a fact; Newton’s theory of gravity explains it.)
Alternatively, the whole thing (evolution by natural selection) is a
fact – with the same veracity as any other in nature. Now if you want to get all postmodernist and say that
nothing is a fact – that everything is personal and subjective – fine. But then we need a new word to distinguish
real “non-facts” (like gravity and evolution) and
non-real “non facts” (like vampires and leprechauns).