No principle is absolutely beyond question, LH, and any scientist worth his salt will say so outright.

But when a sufficient amount of evidence accumulates to indicate something happened and little or no contrary or incompatible evidence survives, then in scientific terminology it is considered a fact. Even facts are not irreversible, but they have a very, very high order of probability of persisting as true. Gravitty -- the mutual attraction of masses -- is not a theory, and neither is evolution. In both cases the suppporting information is consistent enough to warrant considering them both "facts" That's just the way the word is used in science. A theory that attracts enough supporting data gets pormoted to fact-hood, and evolution has long since passed this test at the macro, micro and nano-levels.

If someday it turns out that the apparent mutual attraction of masses porportionate to the inverse of the distance between them, squared, turns out to have actually been caused by a transient belch from the innards of a cosmic-scale dragon who lives inside a black hole at the center of the Universe, and all the proofs line up around this new discovery, the fact will be re-worded to account for it.

Not much probability, though.

A