It's not fact. It is most likely correct, to be sure, but as it stands it is only a scientific theory. Claiming a scientific theory as fact is incorrect. They have wide bases of evidence and substantiation, but are always subject to more studies, more experiments, etc. that may provide more information, more proof, or even disprove the theory, in which case a more accurate theory will, hopefully take their place. The fact that they are testable and falsifiable is what makes them theories. It's semantics, but it's important, since this is part of the foundation of how science works. Evolution's a very good theory, though. Edit: apparently, if one uses the scientific definition of fact, then evolution is one. It is not a fact in the general sense, however. So depending on the definition used, it's both