Admit what you do not know. If you are worried about losing your authority or credibility, then you probably didn't have much to begin with. Admitting what you don't know doesn't weaken you as a presenter, it strengthens you.
It is code for...
"My job is to bring you what I do know, connect that with what you already know, and hopefully create an environment where we can discover some new things together."
That is true power!