If no people believe Y -- literally no people -- then either the topic is very little examined by human beings, or it's very exhaustively examined and seems obvious to everyone. In the first case, I give a smaller probability than in the second case.

In the first case, only X believers exist because only X believers have yet considered the issue. That's minimal evidence in favor of X. In the second case, lots of people have heard of the issue; if there were a decent case against X, somebody would have thought of it. The fact that none of them -- not a minority, but none -- argued against X is strong evidence that X is true.