I've just found a word document, dated 20th May 2007, of a post I wrote about Dark Matter for this thread. I don't think I bothered posting it at the time because of xanadu's bizarre combative attitude on the subject.
Anyways here it is, unfortunately I can't remember the sources I used...
With regards to Dark Matter and the question what it's made up of I think it's best to start with a little history.
The first observational evidence was made way back in 1933 by the Swiss astronomer
Fritz Zwicky. He was studying a small group of seven galaxies in the
Coma Cluster with the objective being to calculate its total mass by studying the dispersion speeds. Zwicky was surprised to find that the dynamic mass was 400 times larger than the luminous mass.
The same phenomenon was again observed in 1936 by Sinclair Smith while calculating the total dynamic mass of the
Virgo Cluster. Although at the time, astronomers had other more important questions to solve, such as the expansion of the Universe, so these observations were put to one side.
In 1970 the question of the existence of Dark Matter reappeared with observations made by
Vera Rubin who was studying the rotation of spiral galaxies. He observed that the stars located at the periphery of spiral galaxies, such as our own and Andromeda, appeared to rotate too fast. In fact the speed remained almost constant when the distance to the center increased. A possible explanation was to think of the existence of a huge non-visible halo of matter (Dark Matter) surrounding the galaxy. To give you an idea how huge this Dark Matter halo is our Sun is about 8.5kpc from the center of our galaxy yet it's estimated the halo is 200-300kpc's wide.
Initially cosmologists suspected Dark Matter was simply made up of Ordinary Matter such as gas clouds and black holes but observations and calculations have proved otherwise.
In the 1990's the Rose Satellite highlighted the presence of gigantic ionized gas clouds within observed galaxy clusters. These clouds seemed to contain ten times more matter that the luminous matter. Unfortunately this turned out not to be the illusive Dark Matter cosmologists had been searching for, in fact these clouds are more proof of the presence of Dark Matter around galaxies. The clouds were very hot and to reach such temperatures the particles that make up the cloud must be accelerated at very high speeds and this acceleration comes from gravity. However the quantity of gas wasn't sufficient to generate such a gravitational field.
Black holes were also another candidate because of their huge mass when compared to our Sun. However calculations showed that one would need almost a million super-massive black holes in a galaxy to fill the lack of matter. With such a large number of black holes the movement of stars in the galactic disc would be strongly amplified, which would make the disk much thicker than what is currently observed. This has lead astronomers to abandon black holes as being the main constituent of Dark Matter.
There are currently two main theories called hot black matter and cold black matter. These theories rely on the mass and speed of the particles composing the dark matter. In the case of "hot" the particles have speeds close to that of light while those of "cold" would be more massive and thus slower. At the moment the best candidate for hot dark matter is the neutrino and the best candidates for cold dark matter are the WIMP and MACHO. Currently, it is the cold dark matter model which seems to be the more consistent however a little bit of hot dark matter is necessary to explain the formation of galaxy clusters.
Another theoretical particle, the axion, would make another good candidate for Dark Matter. This particle would solve problems arising from the antimatter (why matter won over antimatter). Various programs have been launched since 1996 to try to detect axions.
Another possibility is that we need to re-examine the physical laws that constitute the standard model for example some astrophysicists are turning to string theory.