From a scientific perspective,
fact is a colloquialism, so it is really not appropriate to compare it to
theory, which is part of the scientific nomenclature.
The term theory itself is problematic in science, and I would give a general definition of something along the lines of,
a falsifiable, predictive explanation of natural phenomena. One of the most problematic question is the demarcation between a hypothesis and a theory.
So essentially, you can say that the theory of special relativity or the theory of evolution are currently treated in their respective branches of science as facts, but the word fact itself has no specific meaning in science, because the philosophical underpinnings of science hold that any theory, no matter how much it is treated like a "fact", is potentially falsifiable.
One mistake many people make is to think that, in science, a theory is synonymous with "guess" whereas scientists often treat specific theories the way that non-scientists would treat "facts".
If you want further background, you might start here (
http://en.wikipedia.org/wiki/Philosophy_of_science) and consider reading a book or attending a lecture on the philosophy of science, which is a fascinating (and often maddening) subject.