Dark matter

Dark matter refers to material that can't be detected using ordinary techniques, but whose presence can be inferred from gravitational effects on visible matter, like stars and galaxies. As much as 90% of the mass of the universe is believed to exist in this form.

Much of the evidence for dark matter comes from the study of galaxy clusters. Many of these appear to be roughly static and fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. Experimentally, however, it is found to be much greater, often off by an order of magnitude or so, and assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this.

With gravitational theory and new computer analyses, astronomers have now been able to work out where the dark matter is. It is just what you would expect if dark matter and galaxies are clustered in exactly the same way. Galaxies themselves also show signs of being composed largely of dark matter - for instance the rotation curves in and indeed the very existence of our galaxy's disc indicates the presence of a large extended halo.

Knowing where the dark matter is, also reveals how much of it exists. About seven times as much as ordinary matter (that is only one quarter of what is neccesary to slow down the universe's expansion to a halt).

Since it cannot be detected via optical means, the composition of dark matter remains speculative. Large masses like galaxy-sized black holes can be ruled out on the basis of lensing data. Possibilities involving normal baryonic matter include [brown dwarfs]? or perhaps small, dense chunks of heavy elements. The possible amount of baryonic dark matter is, however, restricted by [big bang nucleosynthesis]?. At present, though, the most common view is that dark matter is made of elementary particles other than the usual electrons, protons, and neutrons, such as neutrinos, axions?, or WIMPs.

alternative explanations

An alternative to dark matter is to suppose that gravitational forces become stronger than the Newtonian approximation at great distance. For instance, this can be done by assuming a negative value of the [cosmological constant]? or by assuming modified Newtonian dynamics. Another approach, proposed by Finzi (1963) and again by Sanders (1984), is to replace the gravitational potential with the expression

       GM(1-Be^(-r/ρ))
   U = ---------------
           (1-B)r

where B and ρ are adjustable parameters. However, all such approaches run into difficulties explaining the different behavior of different galaxies and clusters, whereas one can easily describe such differences by assuming different quantities of dark matter.

Do not confuse with white matter or gray matter.

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See [[recent papers on dark matter]].

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Question: article says "this can be done by assuming a negative value of the cosmological constant". Doesn't recent evidence that expansion of universe is accelerating imply a negative value? -- SJK