The physical theory developed between 1900 and 1930 that describes the behavior of matter and energy at atomic and subatomic scales — replacing classical physics in that domain, resisting intuitive interpretation for a century, and underpinning virtually all modern technology from semiconductors to lasers to medical imaging.
Quantum mechanics works with extraordinary precision. Predictions match experiment to ten decimal places. It underlies the transistor, the laser, and MRI machines. And nobody agrees on what it means. The theory that built the modern world remains, after a century, philosophically unresolved.