“I don’t know” is not the end of the viva. It’s not a stain against your name. It doesn’t mean that you automatically lose.
It means you didn’t know something.
If you don’t have information, what do you have? I can think of a few possibilities:
- Probably a question for yourself: you don’t know a definite answer, but what possibilities are there?
- Probably a question or two for your examiners: can you tell me more?
- Definitely a brain, and experience: given the question, given everything you do know, what does that lead you to think?
I don’t know is not the end of the viva. It could be the end of a strand of conversation. Or it could be an opportunity to show how you can think, and engage, discuss and decide. You can give an opinion. YOU can reason things out. I’ll say it again: You’re not here by accident.