There are many candidates for Dark Matter which range from tiny particles of only 10-41 kg (axions) to huge bodies like black holes with masses of up to 1034 kg, but all these candidates can be roughly split into three main categories:
1. B aryonic M atter
Baryonic matter consists of quarks that experience the strong force; particles such as protons and neutrons, and constitutes the matter we see around us every day. The most obvious candidate for dark matter is baryonic matter that does not shine. In order for matter to 'shine', gravitational contraction must occur to create the high temperatures required for thermonuclear fusion to take place in stars. For this to occur, the 'matter collection' (for example a star) must have a minimum mass of about 2x1020 tons. (That's about ten times the mass of Jupiter; the largest planet in our solar system). Possibilities for the baryonic dark matter include brown dwarfs, white dwarfs, planets, burned out stars, neutron stars, and black holes. Such matter might be observed in the effects of micro lensing or in the behaviour of a star held in the gravitational movements of a binary stellar system or binary planetary system with an unseen partner. Such candidates are often called MACHO's (massive astronomical compact halo objects) in contrast to their non-baryonic counterparts, the WIMP's.
2. N on-Baryonic M atter
Non-baryonic matter has two main candidates, the first of these and the more popular of the two are WIMP's (weakly interactive massive particles) which particle physicists have already predicted through Super Symmetry. This would however be very difficult to detect since they vary rarely react with their surroundings. The second possibility is neutrinos, which could constitute some Hot Dark Matter ('hot' referring to their velocity; close to or at the speed of light), but this would require that neutrinos had mass, and cannot account for the small scale structure of the Universe since they are never stationary for long enough! The most likely situation is a variety of different non-baryonic particles mixed together.
3. E verything E lse
There is always the possibility of course that we cannot account for the missing matter because there is none; what if our grasp of forces and their interactions is inaccurate, or entirely wrong? Perhaps the gravitational constant, G, varies with time, or from place to place in the Universe. Perhaps there is another force, or a modification of the gravitational force that changes the way matter interacts over very large distances. There are many such possibilities, but few have any strong arguments, and if they do, how can we hope to progress at all if our fundamental ideas are wrong? It would not make sense to abandon all current research in light of these reflections, and this article will concern itself with the other two categories.
