If a question says
Is there (such a thing) in (place name)?
there are two possible answers: yes or no.
If the answer is yes, then some critical or distrustful users will ask (legitimately) for evidence or a proof. This can usually provided under various forms, and is easy enough to verify as an answer.
However, if the answer is no, how shall it be proved? This is a fundamental and philosophical question.
Hence, what would be the acceptable standards to prove non-existence? Do we need a citation of a site that says it doesn't exist (eg a site saying no penguins in the arctic is easy enough)? Related to this, one might wonder if one should not forbid questions which can potentially lead to such answer?