Quantum fluctuations can cause particles to appear and disappear. Some theories hold that the entire universe was created out of nothing, in a quantum fluctuation. To date, there is no complete and self-consistent theory of quantum gravity, although successful quantum theories have been found for all the forces of nature except gravity. A general term used to describe attempts to quantize gravity. The elementary particle of the gravitational field is the graviton.
String theory is an example of a theory of quantum gravity.
The counter-intuitive weirdness of the concept results in part from the limitations of the particle metaphor in describing a phenomenon that is also in many respects a wave. A mathematical quantity in quantum mechanics whose absolute square determines the probability of a particular process occurring. According to quantum mechanics, it is impossible to have complete and certain information about the state of a physical system, just as a wave cannot be localized to a single point in space but spreads out over many points. This uncertainty is an intrinsic aspect of the system or particle, not a reflection of our inaccuracy of measurement. Consequently, physical systems must be described in terms of probabilities.