It's OK to say: "I don't know."

People commonly believe that a non-answer is better than no answer at all.

However erroneous it would be for me to fall back on appeals of authority, I think there is great wisdom to be had in the following two quotes.

Carl Sagan

I try not to think with my gut. If I'm serious about understanding the world, thinking with anything besides my brain, as tempting as that might be, is likely to get me into trouble.

Socrates

I know one thing, that I know nothing.