If a weight is dropped it falls to the earth. This is a
fact. What best explains the fact is gravity. Likewise,
evolution is a fact and what best explains it is Darwin's
Theory of Natural Selection. All verified "theories" from
gravity to relativity describe certain phenomena that we
know reflect realities. As I've said before,the term
"theory", though useful, has become anachronistic and
terminology always lags behind scientific actuality.
That's a good response. But do you see the difference between the fact of gravity and the "fact" of Darwinian natural selection?
One has been observed, indeed is the very nature of reality. The other has not. Despite years of study, only adaptation within species has been oberved.
Moreover there is an ongoing debate within the scientific commuinty over the mechanism - Darwinian-flavored natural selection vs. punctuated equilibrium. Linneaus (who gave us the categorization system of genus-phylum-species-subspecies) held fervently to the fixity of the species because of the inability (or, sever limitations in the case of donkeys and horses) of interspecial breeding. We now know a dog has 78 chromosomes, for example, while a fox only has 37. They cannot breed (eliminating macroevolution), and deformed outcasts are usually killed by siblings (eliminating "hopeful monster" theory).
You will no doubt recall that Darwin himself, at the end of Origin of Species, expressed his hope that the fossil record would, in the coming years, fill out to provide smooth transitions from species to species. He even stated his theory depended upon this (see last chapter of Origin). This has not happened. For these reasons, it is not honest to call Darwinian evolution a fact.