Well its not really a theory at all, because:
1)it should be consistent with pre-existing theory to the extent that the pre-existing theory was experimentally verified, though it will often show pre-existing theory to be wrong in an exact sense,
2)has survived many critical real world tests that could have proven it false
3)makes predictions that might someday be used to disprove the theory
4)is the best known explanation of the infinite variety of alternative explanations for the same data (Occam's Razor)