“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.
Theories of Dark Energy include:
• As a form of the Cosmological Constant
• A result of Modified Gravity
• Quintessence - a newly proposed dynamical field
• As a result of Time Variations by observers
Currently, none of the proposed theories has enough evidence to be accepted by all theorists.
Note: Unlike Dark Matter, which is believed to exist 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 John Gribbin, pp. 210-211