====== Dark Matter ======

>__The nature of the dominant component of galaxies and clusters remains unknown."__\\ \\ Source : [[http://mnrasl.oxfordjournals.org/content/415/1/L74.full.pdf|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.//

Proponents of Dark Matter theories estimate that it makes up 26.8% of the universe. The rest being [[content:physics:cosmology:dark_energy|Dark Energy]] (68.3%) and visible matter (4.9%).

Since its proposal, various large-scale experimental projects have attempted to 'observe' dark matter<sup> [refs needed] </sup>- 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 //(or Dark Energy)

Alternative theories to explain the galaxy rotation enigma include the so-called [[https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics|MOND]] theory (1983), which proposes that (the force of) gravity is not constant throughout the universe and/or through time. And the [[https://arxiv.org/pdf/2402.19459v1.pdf|'Stochastic Spacetime' ]]theory (2024), in which the spacetime metric is treated as classical, while matter fields remain quantum.

Further reading : [[https://en.wikipedia.org/wiki/Dark_matter|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 [[content:physics:cosmology:missing_baryons|'Missing Baryon Problem']]

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Also see : [[content:physics:cosmology:galaxy_rotation]]

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