What makes a good "the answer is unknown" answer?

Question

In some cases the status of a claim is objectively unknown, e.g. if research indicates that we definitively do not know the answer, or if research leads to no information one way or the other (these are two different cases, mind you).

For example (these are meant to be illustrative and hopefully don't distract from the point, if they are not strong examples just ignore them; full disclosure: I authored these, if linking them is considered poor form feel free to remove links or replace with other examples):

• Research in to "are multivitamins healthy or not" could lead to "it's still open for debate and actively being studied" (or "there are too many variables to ever know", depending on your interpretation).
• Research in to To get husbands, pastor undresses and kisses female church members behinds at beach leads to "we can not know, no information on the primary source is available" (i.e. "the information required to confirm/debunk is not available to us"). This answer has a related discussion regarding its quality, which I still am not sure of.

My question is: What makes a good "we do not / can not know" answer here? How deep / exhaustive should the research be? How can bias be avoided? At what point can we say "we do not know" in a way that satisfies this site's demand for concrete, sourced, accurate answers?

Answer 1

For "absence of evidence" answers, we generally don't accept "Hey, I googled it, and I couldn't find anything relevant." because we can't tell if you searched for the right terms, or maybe the answer is in a book, or in a foreign language, etc.

That makes it very tricky to answer, and this is a quandary frequently faced.

The compromise we have reached, which seems to be widely accepted, is to find a reference to an expert in the field who says they have performed the appropriate literature search and found nothing (or found competing evidence with no clear resolution) and quote them.

Now, this is far from ideal. It suffers from an "Appeal to Authority" fallacy. (That doesn't mean the answer is wrong; it is likely to be right, but there is a chance it is wrong.) However, it seems the best answer we can reasonably hope to give, and it seems more valuable than leaving the question unanswered.