It can be demonstrated today and the historical evidence for its occurrence in the past is overwhelming. Well evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data.
Theories are structures of ideas that explain and interpret facts.
Facts don't go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's in this century, but apples didn't suspend themselves in midair, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered. The final proofs of logic and mathematics flow deductively from stated premises and achieve certainty only because they are NOT about the empirical world. In science 'fact' can only mean 'confirmed to such a degree that it would be perverse to withhold provisional consent'.