Back in my lawyer days I often found it hard to convince witnesses that it was OK to say, “I don’t know.”
But “I don’t know” is a perfectly fine answer. In fact, in many situations it is the only appropriate answer, because it is the only truthful answer.
We all have a tendency, I think, to resist admitting we don’t know something, especially if asked a question to which we think we should know the answer. Of course it’s also true that in normal conversation we don’t expect absolute certainty. We’re usually comfortable answering a question as long as we’re reasonably confident of our answer, even if not absolutely certain of it.
It seems to me that there are probably few things of which we can be absolutely confident. In fact, we ought to be suspicious of any claim of absolute certainty.
So I think we’d probably do well to say “I don’t know” more often.
But I’m not certain of that.