An entity that comprises the majority of the energy of the universe and is responsible for the accelerating cosmic expansion. Its name derives from the inference that it is nonluminous. The nature of dark energy is speculative. Leading theories propose that dark energy is a static, cosmological constant consisting of quantum zero-point energy, and a dynamical condensate of a new, low-mass particle. Alternatively, it has been proposed that the dark energy phenomena are due to a change in the form of gravitation on cosmological length scales. Confirmation of any one of these ideas would have a profound impact on physics. Dark energy appears to be distinct from dark matter, nonluminous particles which make up the majority of the mass of galaxies and galaxy clusters. Determining the nature of dark energy is widely regarded as one of the most important problems in physics and astronomy.