I have been accused of cheating on an online exam because my answers were similar to those in a math website, is this sufficient proof of cheating?
There is literally no proof other than the fact that a website did it the same way I did. Can they say I cheated without any real proof?

There's more proof than a website did it the same way you did. You couldn't explain how you got the answer you wrote. Besides, &quot;real proof&quot; is a vague term. From their perspective it could very well be real proof, even if from yours it isn't.
You probably should get ready to defend yourself. See the answers in this related question.