In the standard lambda-CDM model of cosmology, the mass–energy content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as dark energy.[4][5][6][7] Thus, dark matter constitutes 85%[a] of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content.[8][9][10][11]
Each suggested kind of dark matter is classified as "cold", "warm", or "hot" according to velocity (more precisely, its free streaming length). Recent models have favored a cold dark matter scenario, in which structures emerge by the gradual accumulation of particles.
Although the astrophysics community generally accepts dark matter's existence,[16] a minority of astrophysicists, intrigued by specific observations that are not well explained by ordinary dark matter, argue for various modifications of the standard laws of general relativity. These include modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity. So far none of the proposed modified gravity theories can describe every piece of observational evidence at the same time, suggesting that even if gravity has to be modified, some form of dark matter will still be required.[17]
^Carroll, Sean (2007). Dark Matter, Dark Energy: The dark side of the universe. The Teaching Company. Guidebook Part 2 p. 46. ... dark matter: An invisible, essentially collisionless component of matter that makes up about 25 percent of the energy density of the universe ... it's a different kind of particle... something not yet observed in the laboratory ...
^Ferris, Timothy (January 2015). "Dark matter". Hidden cosmos. National Geographic Magazine. Archived from the original on 25 December 2014. Retrieved 10 June 2015.
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