Mekhi has a new post about dark matter and dark energy at her rationalisingtheuniverse blog HERE – excerpts below:
“Roughly 68% of the universe is thought to be dark energy making it whoppingly the largest constituent out there. Dark energy is characterised in terms of its energy density and pressure, both negative. Its existence is thought to be cause for the accelerated expansion of the universe. We know how much is out there because we know how it effects the universe’s expansion but apart from that it is shrouded in mystery. There are few strands of thought at which we can grasp, the first is that dark energy is a property of space. As we’ve learn from the post on fields, empty space is not ‘nothing’, space is the gravitational field and can possess energy. If dark energy is a property of space itself and is the cause of expansion, then as the universe expands, more space would come into existence, more energy would appear and as a result this energy would continually cause the universe to expand faster and faster. In this case the dark energy also has a pseudonym – the cosmological constant, a constant energy density filling the space homogeneously which Einstein wrote into his equations for general relativity. (Although the energy increases as space expands, so of course does space, keeping the energy density (energy per unit volume) constant.
Another explanation for how space comes to acquire energy is from the formation of particle, anti-particle pairs in quantum theory. Quantum theory says that particle, anti-particle pairs can be spontaneously created from vacuum fluctuations because their overall mass is zero (anti-particles have negative mass). Perhaps this is source of the energy. Or finally, perhaps dark energy is new kind of ‘fluid’ or field that fills all of space but whose effect on the universe is opposite than that of normal energy and matter. Instead of being attractive due to gravity, this fluid or field is repulsive, causing the outward expansion of the universe. This new kind of fluid or field has been given the name ‘quintessence’ a Greek name for a fifth element or fundamental force. Fields seem to be at the heart of everything.”
Wikipedia explains here:
“The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum. Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter, gravitational lensing, its influence on the universe’s large-scale structure, and its effects in the cosmic microwave background. Dark matter is transparent to electromagnetic radiation and/or is so dense and small that it fails to absorb or emit enough radiation to be detectable with current imaging technology.
The standard model of cosmology indicates that the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Thus, dark matter constitutes 84.5% of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content. The great majority of ordinary matter in the universe is also unseen, since visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass-energy density of the universe.
The dark matter hypothesis plays a central role in current modeling of cosmic structure formation and galaxy formation and evolution and on explanations of the anisotropies observed in the cosmic microwave background (CMB). All these lines of evidence suggest that galaxies, galaxy clusters, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals.
Although the existence of dark matter is generally accepted by most of the astronomical community, a minority of astronomers argue for various modifications of the standard laws of general relativity, such as MOND, TeVeS, and conformal gravity that attempt to account for the observations without invoking additional matter.
Many experiments to detect proposed dark matter particles through non-gravitational means are under way. On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse galaxy (UDG) with the mass of the Milky Way galaxy, but with nearly no discernable stars or galactic structure, may be made almost entirely of dark matter.“