When I teach my introductory (and free) Successful Technical Speaking class to try to help beginners enter the world of speaking, what would you guess is the number one road block? Surprisingly it's NOTT the fear of public speaking, it's the fear that they won't be able to answer all the questions and will look 'stupid'. One of the questions I ask them at this point is 'does it bother you when you attend a presentation and the speaker isn't able to answer a question?'. I've not yet had anyone answer yes, because they realize that's an impossibly high standard. If a speaker can't answer a dozen questions, that's a different story - but rare.
Something about human nature wants to hold ourselves to a higher and unreasonable standard than we do everyone else, and just pointing that out moves a lot of people past this common road block. Interesting isn't it?
What attendees do look for is poise, do you melt down when you hit a question you can't answer (whether it's in scope or not), or do you handle it gracefully. I'm not fond of the 'I'll look that up and back to you' answer, I prefer to stick with a simple 'I don't know the answer to that' as my answer. I'm experienced enough to know that I'll probably NEVER know all the answers, and humble enough to openly admit it when someone asks me a question I don't know. No covering up, no bluster, just - I don't know. Think about it the next time you attend a presentation. Regardless of response, poise is the key.
