Evolution is both a fact and a theory. This was argued well by Steven Gould some years ago. The big problem is not that science claims evolution is a fact but that the creationists use a definition of theory that has no basis in science.

The common usage of theory is a bastardization of hypothesis. Just as they assemble, parade around, and finally burn their straw man evolutionary ladder, while ignoring the complex lattice of common descent, they portray theory as simple speculation, barely above a guess and well below law on the ladder of epistemology instead of the complex amalgam of hypothesis, sub-theories and laws that make up the theory of evolution.