Standard physics says the Big Bang produced matter and antimatter in equal quantities, which means they should have annihilated each other completely, leaving nothing. Something broke that symmetry, and we're living in the results, and as far as I can tell no one actually knows what that something was.
Something I keep wondering: does the asymmetry have to be explained by something that happened at or before the Big Bang, or could a process after it have caused it? I genuinely don't know if that's a meaningful distinction or not.
I've had a version of this question in the back of my head since second-year physics. Every time someone explains baryogenesis to me I nod along and then realize I still don't have anything that feels like an actual answer. I wonder if part of the issue is that the question keeps getting reframed in terms of what we can measure rather than what would actually resolve it.
What's strange about this one is that it sounds almost philosophical but it's not. The asymmetry is a measurable physical fact. The question is just what caused it, which in principle you could get an answer to. Nobody's figured out how yet.