Quantum mechanics, cosmology, and the nature of reality.
Quantum mechanics describes the behaviour of particles at the smallest scales. The counterintuitive results — superposition, entanglement, the observer effect — follow consistently from well-tested equations. The weirdness is real, not metaphorical.
The Standard Model describes all known fundamental particles and three of the four fundamental forces. It's extraordinarily predictive — the most precisely tested theory in all of science — and everyone suspects it's incomplete.
Dark matter constitutes roughly 27% of the universe's mass-energy content. We know it exists because of its gravitational effects. We have no idea what it is. Dark energy, comprising approximately 68%, is even more mysterious.