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 giant '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 via gravity. The proposed Dark Matter particles from which the halo is made have been named 'Weakly Interacting Massive Particles' (WIMPs). Further calculations show 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.

Dark Matter theorists estimate that it 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 responsible for its accelerated expansion. (68.3%) and visible matter (4.9%).

Since its proposal, various large-scale experimental projects have attempted to 'observe' dark matter - or more accurately its effects, as it would need to be essentially 'invisible' to have escaped detection up until now.

A notable experiment is the LUX-ZEPLIN (LZ) detector at the Sanford Underground Research Facility in South Dakota, US. The purpose-built facility is 1km underground (to avoid interference from other particles) and uses several tonnes of liquid Xenon in its detectors. So far, it has not detected a single 'Dark Matter' event.

To date, no experiment has been able to unequivocally detect any Dark Matter whatsoever. Leading some theorists to begin to doubt the existence of the Weakly Interacting Massive Particles which are presently proposed to be its key component.

Alternative theories to explain the galaxy rotation enigma include the so-called MOND theory (1983), which proposes that (the force of) gravity is not constant throughout the universe and/or through time. And the 'Stochastic Spacetime' theory (2024), in which the spacetime metric is treated as classical, while matter fields remain quantum.

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 the universe should contain as a consequence of the Big Bang (using the maths of the Standard Model), the result does not correlate with actual cosmological observations.

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, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. and 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 responsible for its accelerated expansion.

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