We can see stars directly. Some are big, some are small (relatively speaking), some are bright, some less so, but so long as nothing is in the way, they’re there. We can’t see black holes directly. They’re tricky, difficult to describe maybe, possibly destructive if we get too close and you don’t want your examiners to talk about them-
-oh, yeah, this is a thesis metaphor!
The stars are your contributions. They’re in your thesis, and so long as nothing is in the way (clunky writing, obscure terminology, confusing structure) your examiners and anyone else reading your thesis will see them. They might still have questions about them, but they will see clearly that there is something valuable there.
The black holes are things you can’t see clearly. Problems, Issues, Gaps – things you don’t want your examiners to ask about because it’s hard to talk about them. And you worry that once you’re in the conversational gravitational pull you won’t be able to escape the crushing forces at the heart of the matter!
The stars and black holes in your thesis are made up of the same stuff though: ideas.
Get back to ideas in your preparation. Stars or black holes, what are the ideas that make them up? Why do they matter in their respective way? How can you best describe them?
Get used to the brightness of your stars. Grow comfortable being in orbit of your black holes.