It's simple: if a fact is something that is indisputably true, no scientific theory, even if it is true, can be case. This is because scientific theories are by their definition testable and falsifiable. They can be built upon and changed. If it is not falsifiable, it isn't a scientific theory. Even if evolution or gravity are never disproven, the possibility still exists. And as such, they are disputable, and therefore not facts. If the original statement was "it's not even a debate, evolution governs the development of life", I would have less of a problem with it. It would be even better of it was "it's not even a debate, evolution is the single best explanation we have on the origins of life", but that isn't nearly as simple a statement.