Wave-particle duality means tiny things—like electrons and atoms—behave as both waves and particles. In the double-slit experiment, a single particle, when not observed, forms an interference pattern as if it travels through both slits at once. But when we observe it, the pattern disappears, and it behaves like a particle.
Some physicists use this to argue for the multiverse. Instead of a wavefunction “collapsing” when we observe it, the many-worlds interpretation says all possibilities happen—but in different universes. One universe sees the particle go left, another sees it go right.
Does this prove a multiverse? No. But it’s one of the biggest reasons some physicists think we live in a reality bigger than we can see.