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 if the outer regions were rotating at the observed speed, they would fly away from the centre and the galaxy would disintegrate.

As a means of explaining the anomaly, the idea of Dark Matter was proposed. For the explanation to work, the galaxies would have to be surrounded by a 'halo' of Dark Matter - a previously unknown form of matter that has mass, and yet does not interact with 'ordinary' matter in any other way than by gravity. Further calculations showed that for the galaxies to hold together, there would need to be approximately five times more Dark Matter (in terms of mass) than standard matter.

The current estimates are that Dark Matter makes up 26.8% of the universe. The rest being Dark Energyplugin-autotooltip__plain plugin-autotooltip_bigDark Energy

"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… (68.3%) and visible matter (4.9%).

Since its proposal, various experimental projects have attempted to 'observe' dark matter ^{[refs needed]} - or more accurately its effects, as it would need to be 'invisible' to have escaped detection up until now. To date no experiment has been able to directly and unequivocally detect Dark Matter.

Alternative theories to explain the galaxy rotation enigma include the so-called MOND theory, which proposes that (the force of) gravity is not constant throughout the universe and/or through time.

Further reading : Wikipedia

Note that of the 4.9% of 'ordinary matter' (a.k.a. 'Baryonic matter') less than half of that is currently accounted for. This is the so-called 'Missing Baryon Problem'plugin-autotooltip__plain plugin-autotooltip_bigMissing baryons problem

Baryons are the sub-atomic particles primarily responsible for an object's mass. They are named after the Greek word βαρύς (barýs) for 'heavy'. They include protons and neutrons. When cosmologists calculate the amount of baryonic matter which t…

Also see : Galaxy Rotation Problemplugin-autotooltip__plain plugin-autotooltip_bigGalaxy Rotation Problem

"The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, a…

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