Dark energy began to dominate about 5 billion years ago, corresponding to the time when the expansion of the Universe began to accelerate.
The proportions of dark energy, dark matter, and ordinary matter in the universe, according to the latest observations.
Dark energy is a property of empty space, allowed by Einstein's general theory of relativity, that acts as a large, negative pressure and pushes the Universe apart at a faster and faster rate. Its existence has been hypothesized to explain observations which indicate that the expansion of the Universe is accelerating. The most recent observations by Planck (a European Space Agency mission) suggest that 68.3% of the mass-energy of the Universe is in the form of dark energy. A further 26.8% consists of dark matter; the remaining 4.9% is ordinary matter.
The exact nature of dark energy has yet to be established. However, dark energy is believed to be extremely homogeneous (evenly spread throughout space) and of very low density, roughly 6.9 × 10-29 kilogram per cubic meter. This is much lower than the density of ordinary matter or energy with the Milky Way Galaxy and makes it probably undetectable in laboratory experiments.
Various models of dark energy have been proposed, including a cosmic field associated with inflation; a different, low-energy field called quintessence; and the cosmological constant, or vacuum energy of empty space. Unlike Einstein's famous fudge factor, the cosmological constant in its present incarnation doesn't delicately (and artificially) balance gravity in order to maintain a static universe; instead, it has negative pressure that causes expansion to accelerate.
Discovery of dark energy
Until the late 1990s, astronomers were convinced that the expansion of the Universe had to be slowing: it might slow and eventually reverse if the overall energy density were high enough, or it might slow but continue forever. It was true that no slowing had been observed but the assumption seemed reasonable since gravity was the only significant force known to be acting over cosmic distances. Then, 1998, a startling discovery was announced. Observations of remote supernovae by the Hubble Space Telescope showed that, billions of years ago, the Universe was expanding more slowly than it is today. To everyone's surprise the rate at which the Universe is expanding has increased – a fact that could only be explained if there were a previously unknown phenomenon acting to pull the Universe apart. For more details, see the separate entry on the discovery of dark energy.