"...it's just a theory..."

You are arguing from ignorance, Nigel. The scientific process's finest achievements are its theories. Theories are the best explanations we have for the phenomena we observe. They are accumulations of evidence and interpretations thereof from as many angles as possible. Science does not concern itself with proofs. In fact, the best science always challenges notions that looks like they're in danger of reaching triumphalist certainties. You may be certain that certain phenomena occur. I can touch my fingertip and thumb tip together, that much is as certain as certain can be (you could always ask me for a video). But knowing that is not science. All the science is in the observation and the explanation of evidence. And there's always someone waiting to turn your notions on their heads. That's how human knowledge expands. There's no other way.