Have you ever been around someone and no matter what they’re asked, they have an answer? And sometimes you know better, and when they answer authoritatively and definitively you end up doubting all their answers because you realize they’re never going to say ‘I don’t know.’
It’s counter-intuitive. Never saying ‘I don’t know’ leads people to realize you really don’t. And admitting you don’t know gives more credibility to things you do.
Another way to say ‘I don’t know’ is to answer with questions – more and more questions. In answering your questions, the person asking the original question may end up answering it. So it will have the same effect as if you did know even though you didn’t. It’s a nice deal -- they get something better than your flailing around can give and you keep your credibility.