In the 1970s, physicists uncovered a problem with the Standard Model of particle physics—the theory that describes three of the four fundamental forces of nature (electromagnetic, weak, and strong interactions; the fourth is gravity). They found that, while the theory predicts that a symmetry between particles and forces in our Universe and a mirror version should be broken, the experiments say otherwise. This mismatch between theory and observations is dubbed 'the Strong CP problem'—CP stands for Charge+Parity. What is the CP problem, and why has it puzzled scientists for almost half a century?