For many questions, "we don't know" is the correct answer - it would be beyond foolish to seek to ban such answers. Of course, a plain "we don't know" without saying anything more than those three words would probably make for a bad answer, just like any other unsupported three-word answer. Here are some ideas I once came up with for possible ways to make a "we don't know" answer good:
- show your work. Give list of Google queries you used, or resource sites/pages checked, or terms searched in e-books.
- list all the information we do know ("there's no mention of xyz - the closest is this mention of yxz on page 73")
- give some sort of explanation for why it's reasonable that we don't know ("it wouldn't make sense for them to tell us, because blah blah blah" or "people have written fanfic taking xyz in this direction, so it's unlikely to be confirmed as going in that direction anywhere in canon")
The basic principle, as with many SE answers, is to back it up if at all possible. Of course it tends to be hard to prove a negative, but any sort of extra knowledge - even just a deep personal experience of the topic - can be helpful. If you say "I've searched through all the books and also supplementary information on websites X, Y, and Z, and none of them say anything about this", that's a much more respectable answer than "dunno, mate".