This is, by the way, the major weakness with Physicalism as an ontology; the scientific method is a great methodology, but it would be foolish to say "the only things that exist are those described by science" since science is, by its very nature, incomplete and continually evolving. One could say, of course, that the only things that exist are those described by some future perfect physics, but this claim is kind of circular/meaningless.

This issue is known as Hempel's dilemma: http://en.wikipedia.org/wiki/Hempel's_dilemma