What kind of job or position can you get, where you routinely say “I don’t know” and it’s completely acceptable? Probably not many. The point of most jobs is to know everything about what that position entails, and to then use that knowledge to answer any questions or problems that arise.
But as a grad student, you’ll probably find that you get asked questions where the only honest answer you have is “I don’t know.” And, you know what? That’s perfectly okay. It’s even better when you follow it up with “but I can look it up” or “but I think maybe it has to do with…” or “based on what I’ve read…” That’s the neat thing about original, cutting edge research – the answer is rarely clear or easy.
Even when you’re in your defense, at the end of the day, every question that you get asked doesn’t need to be properly answered. Each committee member will likely have a set of questions they want to ask you. They will have some “easy” questions that they’ll be very concerned if you can’t answer. They’ll have some “medium” questions that are there to test that you have the actual level of knowledge that they expect you to have to get the degree. And then there will be “hard” questions that are used to test how far your knowledge goes. These are the questions you might get where you may have to answer with “I don’t know.” But that’s okay, the point of them is to test where your knowledge on the subject ends.
I think this concept have not having to know everything up front is really kind of cool. And it’s kind of another plus for grad school. I think it also helps breed research skills. By having to learn the limits of you knowledge I think you become more curious and willing to strive forward and figure out how to extend your knowledge.