Yes, it's just a theory. But it's a theory that is widely supported by evidence gathered over time. It's the best we've got for now, and it works for all purposes that we need it to. Maybe we'll find out that we're wrong, and I'd never say that we won't be proven wrong ever, but I don't think that it'll be anytime soon. If we can go faster than light, good for us, but I don't think that it'll happen (provided it CAN happen) any time in the next 100 years AT LEAST.