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dark energy

dark energy proportion
The proportions of dark energy, dark matter, and ordinary matter in the universe, according to the latest observations
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 10-29 grams per cubic centimeter.

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.

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