It sounds like you're using "theory" wrong, although in the case of the big bang, I think the scientists also used it wrong. General Relativity is a theory because it has overwhelming evidence to support it - its the opposite if speculative. But the big bang should never have been called a theory because it only has two pieces of evidence, both of which also need supporting evidence to be interpreted (red shift and cosmic background radiation). Now there's evidence against the big bang, and scientists really ought to be feeling quite foolish for ever calling it a theory.