Quantum foam is a quantum fluctuation of spacetime on very small scales due to quantum mechanics. At this small scale, virtual particles of matter and antimatter are constantly created and destroyed. The uncertainty principle allows these virtual particles to spontaneously emerge from zero-point energy fluctuations of spacetime. If the quantum foam is a "sea" of fluctuating spacetime, virtual particles are like ripples on that sea.
John Wheeler, who devised the concept in 1955, suggested that the uncertainty principle might imply that over sufficiently small distances and sufficiently brief intervals of time, the "very geometry of spacetime fluctuates". These fluctuations could be large enough to cause significant departures from the smooth spacetime seen at macroscopic scales, giving spacetime a "foamy" character.
The fluctuations occur at the Planck length (≈ 10−35 m), but some models of quantum gravity predict much larger fluctuations. X-ray and gamma-ray observations of quasars show no detectable degradation at the farthest observed distances, implying that spacetime is smooth at least down to distances 1000 times smaller than the nucleus of a hydrogen atom, implying that the foam could be attoscale.