No, demons don't prove God's existence. God proves God's existence. It's possible that an authentic experience with a supernatural force or being would be evidence to you that spirit beings or forces exist, but that wouldn't prove the existence of God, just that there are more realities than just material ones.

> I know many people who have had supernatural experiences. Intense, unexplainable ones, such as seeing someone/something in your room that is not really there... What other explanation is there? What arguments are there against that?

People give psychological explanations, such as episodes of various forms and manifestations of schizophrenia or psychosis.

I just randomly had this thought. If a non-believer were to be curious and tried things like ouija boards or did something to purposely encounter a demon or an evil spirit, and they see one or experience something supernatural, does that not prove God’s existence? I know many people who have had supernatural experiences. Intense, unexplainable ones, such as seeing someone/something in your room that is not really there... What other explanation is there? What arguments are there against that?