In an extract from his upcoming book, The Greatest Show on Earth (that I am eagerly awaiting to read) Richard Dawkins explains that it’s the ‘proof beyond doubt’ that established a scientific theory as a “fact” – unlike, say in Mathematics, where a rigorous, impeccable, ultimate proof is (a) possible, and (b) required to prove a theorem.
Why, then, do we speak of “Darwin’s theory of evolution”, thereby, it seems, giving spurious comfort to those of a creationist persuasion — the history-deniers,  — who think the word “theory” is a concession, handing them some kind of gift or victory? Evolution is a theory in the same sense as the heliocentric theory. In neither case should the word “only” be used, as in “only a theory”. As for the claim that evolution has never been “proved”, proof is a notion that scientists have been intimidated into mistrusting.
Influential philosophers tell us we can’t prove anything in science.
Mathematicians can prove things — according to one strict view, they are the only people who can — but the best that scientists can do is fail to disprove things while pointing to how hard they tried. Even the undisputed theory that the Moon is smaller than the Sun cannot, to the satisfaction of a certain kind of philosopher, be proved in the way that, for example, the Pythagorean Theorem can be proved. But massive accretions of evidence support it so strongly that to deny it the status of “fact” seems ridiculous to all but pedants. The same is true of evolution. Evolution is a fact in the same sense as it is a fact that Paris is in the northern hemisphere. Though logic-choppers rule the town, some theories are beyond sensible doubt, and we call them facts. The more energetically and thoroughly you try to disprove a theory, if it survives the assault, the more closely it approaches what common sense happily calls a fact.
Read the whole thing here.
Also see my earlier post Science Never Proves Anything where I argue how all scientific truth is provisional.