Dear friends,

(Relativity and quantum mechanics have been around for nearly a century, yet they remain confusing in some sense even to those who understand these theories well. We know they’re correct because they’ve been tested so thoroughly in so many ways. But they still don’t make sense.)

What does the last sentence "they still don’t make sense" mean? ("they" refer to "Relativity and quantum mechanics", I think.)

Thank you very much.
"'They" refers to "these theories". Even though we know that these theories are correct, we don't understand why Emotion: smile
Yes, I agree with u that "they" refer to "R&Q mechanics"

But they still remain confusing to some extent for us to explain clearly.