You see, 'theory' is the key word here. It's just a theory which hasn't been proven yet.
Evolution is Not Just a Theory: home
The Theory of Evolution is a theory, but guess what? When scientists use the word theory, it has a different meaning to normal everyday use. That's right, it all comes down to the multiple meanings of the word theory. If you said to a scientist that you didn't believe in evolution because it was "just a theory", they'd probably be a bit puzzled.
In everyday use, theory means a guess or a hunch, something that maybe needs proof. In science, a theory is not a guess, not a hunch. It's a well-substantiated, well-supported, well-documented explanation for our observations. It ties together all the facts about something, providing an explanation that fits all the observations and can be used to make predictions. In science, theory is the ultimate goal, the explanation. It's as close to proven as anything in science can be.
In everyday use, theory means a guess or a hunch, something that maybe needs proof. In science, a theory is not a guess, not a hunch. It's a well-substantiated, well-supported, well-documented explanation for our observations. It ties together all the facts about something, providing an explanation that fits all the observations and can be used to make predictions. In science, theory is the ultimate goal, the explanation. It's as close to proven as anything in science can be.