Everything that exists is evidence of causality, I am a necessary result of the chain of causality (starting with the "big bang"), hence, by observing everything, you can deduce that I exist. The only problem is that everything only exists in the mind, or consciousness, and consciousness does not physically exist; but at least in the context of consciousness, I exist. This also means that outside the context of consciousness, if I don't exist, you don't exist either. Within the context of your consciousness; I exist, as you within mine. There I proved to you that I exist.

Except that's not existence. Perception is not existence. Thoughts, illusions, and hallucinations are not existence. Rather, they are another form of reality. To appear to exists while not existing. This is one possible form.

You're just mad that I'm right.