There is a great tendency by people who believe in a “God” when asked to for proof that their particular god exists often answer, ‘I feel it in my heart’ or ‘look around, the universe is so complex that it could not happen by chance’. When you point out that such answers are opinions rather than proof they request that you cannot proof that there is no god. The same thing happens with people who spot an UFO declare that it must be aliens from another planet or time travelers and then demand that you proof that it isn’t so. This kind of reasoning is called a logical fallacy (argumentum ad ignorantiam). The implication of ignorance is not meant to be an insult here, it means merely a lack of knowledge. One is arguing that because the lack of knowledge one can draw a conclusion from that. This is a kind of false dichotomy, which indicates that there perhaps may have been insufficient investigations and there is not enough information to prove the proposition to be true or false and excludes other options. This kind of fallacy happens often when one proposes that there are only two possibilities and it becomes a false ‘true or false’ question. The demand to prove the negative is therefor invalid. In the statement ‘God exists’ one can substitute any other word or sentence such as Santa, Thor, the supernatural, Zeus and so on and then demand proof that they don’t. One cannot draw a conclusion based on lack of knowledge.