Unanswered Questions
The lack of an answer in your thesis or in the viva doesn’t mean a problem for you.
Perhaps it tells you something about the question. There’s a reason you don’t know. What is it? That’s important.
Or maybe focus on why you can’t give an answer directly. What’s missing? Where’s the gap? That will be important too.
An unanswered question might not be unanswerable forever. Discussing “why you can’t give an answer now” could be the best response you could give in the viva.