Predicted by Dirac in 1931 -- a single magnetic pole without its opposite. Every magnet you have ever held has north and south. Break it in half, you get two smaller magnets, each with both poles. The monopole is the one that breaks this rule. The ghost that haunts electromagnetic theory.
Maxwell's equations look almost perfectly symmetric between electricity and magnetism. Almost. The missing piece is the magnetic charge. If monopoles exist, that symmetry becomes exact. Everything snaps into place. The universe makes more sense with monopoles than without them.
February 14, 1982. Blas Cabrera's SQUID detector at Stanford recorded a single, perfect signal consistent with a magnetic monopole passing through the device. The signal was never reproduced. Valentine's Day, and physics got ghosted.
monopole.boo