A hypothetical form of energy that fills all the space and tends to increase the rate of expansion of the Universe. Assuming the existence of dark energy is a way to explain recent observations that the Universe appears to be expanding at an increasing rate. Dark energy seems to be a kind of anti-gravity force and is supposed to be related to vacuum energy. Where gravity pulls things together at the more local level, dark energy tears them apart on the grander scale. The acceleration equation, one of Einstein's equations for the homogeneous Universe, indicates that if the Universe is accelerating, the pressure of the driving component should be strongly negative. The first indication of dark energy was provided by the observation of Type Ia supernovae. Other probes of dark energy are: baryon acoustic oscillations, weak gravitational lensing, and clusters of galaxies. In the standard model of cosmology, dark energy currently accounts for almost 74% of the total mass-energy of the Universe. Two proposed forms for dark energy are the cosmological constant and exotic component such as quintessence.