The universe. The Big bang, time, quantum mechanics. Is our universe infinite? Is it the only universe? Did the Big bang start ours and will it end with a big crunch and will that collapse just cause a big bang that repeats and if so what iteration of that cycle do you suppose we are in? And does each universe behave the same, similar laws and physics and such? Stars, planets, etc?

Deconstructing from religion. It was a lot. I'm better now, but being stuck in it all was overwhelming and was like being in an existential crisis every day until it ended. I just went along with it and kept it all inside for decades and it wasn't fun.

Consciousness and our sense of self. Is consciousness an illusion? What even is "me"? It includes all the gut bacteria and mitochondria with different DNA than us and our brains are these amazing pattern recognition machines that also have abysmal memory storage and recall, but can notice the tiniest of nuance sometimes, but also can't remember where we put the thing we were just holding 2 minutes ago. And all the while our brain is confidently telling us "I am me" and is processing all the inputs like sights and sounds and interpreting all that into what we think we see and what we think we heard. But did we? How would we know if upon seeing the color red our brain interprets that as blue and we confidently declare we see red.