The observational results of the latest 15 to 20 years have established a standard model for the cosmology which has some amazing consequences. A mysterious entity, the dark energy, has been confirmed as the dominant component of the Universe, and is also responsible for its accelerated expansion. However, its physical nature remains unknown. Unveiling the nature of the dark energy is one of the main problems of cosmology."

Source : The Dark Energy Survey: Status and First results Nuclear and Particle Physics Proceedings Volumes 273–275, April–June 2016, Pages 302–308, 37th International Conference on High Energy Physics (ICHEP)

Dark Energy is currently believed to make up 68.3% of the universe, and is the most accepted hypothesis to explain various observations (beginning in the 1990s) indicating that the universe is expanding at an accelerating rate.

Note that the speed at which it's expanding has not yet been determined to an acceptable degree of accuracy - with different observation techniques apparently showing widely differing rates.

Theories of Dark Energy include:

• As a form of the Cosmological Constant (see link below)
• A result of Modified Gravity
• Quintessence - a newly proposed dynamical field
• As a result of Time Variations by observers etc

Currently, none of the proposed theories has enough evidence to be accepted by all theorists.

Note: Unlike Dark Matterplugin-autotooltip__plain plugin-autotooltip_bigDark Matter

"The nature of the dominant component of galaxies and clusters remains unknown."

Source : Measuring the dark matter equation of state (Mon. Not. R. Astron. Soc. 415, L74–L77)"

In the 1930s, astronomical observations of galaxy rotations showed that the outer regions were rotating (about the galaxy's 'centre') at the same speed, or faster, than the central regions. Subsequent calculations referring to the galaxy's mass, and thus its internal gravitational attractions, showed that i…, which many comologists believe exists in so-called 'halos' around galaxies, Dark Energy would necessarily pervade every part of the entire universe - but no effects have ever been detected in any experiments on Earth, or in our 'local' space research. The calculated 'push' of dark energy, however, is very low :

The amount of dark energy contained in the volume of the entire Earth is about as much as the amount of electricity used by the average US citizen every year […]"

Source : ' 13.8 ' by Dr. John Gribbin, pp. 210-211

Also see Cosmological Constant Problemplugin-autotooltip__plain plugin-autotooltip_bigCosmological Constant Problem

The cosmological constant problem is the disagreement in 'measured' values of the cosmological constant and theoretical calculations of its value.

The cosmological constant was first proposed by Einstein as a mathematical necessity to rationalise how the universe has resisted collapsing under its own gravity. General relativity, however, predicts the

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