"Many scientists and philosophers of science have described evolution as fact and theory, a phrase which was used as the title of an article by Stephen Jay Gould in 1981. He describes fact in science as meaning data, not absolute certainty but "confirmed to such a degree that it would be perverse to withhold provisional assent." A scientific theory is a well-substantiated explanation of such facts. The facts of evolution come from observational evidence of current processes, from imperfections in organisms recording historical common descent, and from transitions in the fossil record. Theories of evolution provide a provisional explanation for these facts."
"Fact is often used by scientists to refer to experimental or empirical data or objective verifiable observations. "Fact" is also used in a wider sense to mean any theory for which there is overwhelming evidence.
A fact is a hypothesis that is so firmly supported by evidence that we assume it is true, and act as if it were true. —Douglas Futuyma
Evolution is a fact in the sense that it is overwhelmingly validated by the evidence. Frequently, evolution is said to be a fact in the same way as the Earth revolving around the Sun is a fact. The following quotation from H. J. Muller, "One Hundred Years Without Darwin Are Enough" explains the point.
There is no sharp line between speculation, hypothesis, theory, principle, and fact, but only a difference along a sliding scale, in the degree of probability of the idea. When we say a thing is a fact, then, we only mean that its probability is an extremely high one: so high that we are not bothered by doubt about it and are ready to act accordingly. Now in this use of the term fact, the only proper one, evolution is a fact.
The National Academy of Science (U.S.) makes a similar point:
Scientists most often use the word "fact" to describe an observation. But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples. The occurrence of evolution in this sense is fact. Scientists no longer question whether descent with modification occurred because the evidence is so strong.
Gould also points out that "Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory—natural selection—to explain the mechanism of evolution." These two aspects are frequently confused. Scientists continue to argue about particular explanations or mechanisms at work in specific instances of evolution, but the fact that evolution has occurred and is still occurring is undisputed.
A common misconception is that evolution cannot be observed because it all happened millions of years ago and the science does not therefore depend on facts (in the initial sense above). However both Darwin and Wallace, the co-founders of the theory, and all subsequent biologists depend primarily on observations of living organisms; Darwin concentrated largely on the breeding of domesticated animals whereas Wallace started from the biogeographical distribution of species in the Amazon and Malay Archipelago. In the early twentieth century, population genetics had centre stage, and more recently DNA has become the main focus of observation and experimentation."