it can also refer to explanatory theories (such as
Charles Darwin's theory of
natural selection) which explain the mechanisms of evolution. To a scientist,
fact can describe a repeatable observation that all can agree on; it can refer to something that is so well established that nobody in a community disagrees with it; and it can also refer to the truth or falsity of a proposition. To the public,
theory can mean an opinion or conjecture (
e.g., "it's only a theory"), but among scientists it has a much stronger connotation of "well-substantiated explanation".