A couple of things for me:

What is the significance of the monster group ? Is there some upper limit on the symmetry of the universe ?

Is the Riemann hypothesis true ? This looks weirdly abstract but has some significance for cryptography. It also seems part of intuitive but is one maths hardest problems to solve!

What the hell are black holes in the context of space time and reality ? I watched this really good interview of Brian Cox by Cloe Abrams. I was sort of semi comfortable with what I thought they were, but after watching this I am more profoundly confused by them than before

Dark energy/matter… ok. I stop. your turn.