Theories of Dark Matter and What its Made of
Currently we have two main theories to describe the nature of dark matter. One is hot dark matter and another one is cold dark matter.
According to the hot dark matter model the particles have speeds close to light and the speed of these partilces is crucial to the Big Bang model of cosmology and the formation of our universe.The best candidate to constitute the hot dark matter is the neutrino. Neutrinos have a very small mass, do not interact via either the electromagnetic or the strong nuclear force and are therefore very difficult to detect.
On the other hand, according to the theory of cold dark matter, the particles that constitute the cold dark matter are more massive than that of hot dark matter and thus slower.The candidates for cold dark matter are WIMP's (weakly interacting massive particles - heavy neutrino, neutralino, axion) and MACHO's (massive compact halo objects - black holes, white dwarf, brown dwarf etc.).
It is strange that the theories start from two ends. If the universe is primarily made of hot dark matter then the largest structures are formed initially and the divided to smaller structures. For example, at first super clusters of galaxies formed, then they divided into galaxy clusters, then into galaxies and then into smaller structures. But if cold dark matter is the primary component of the universe then the smaller structures gathered to form larger structures, totally opposite to hot dark matter.
But which theory is the correct one ? Cold or hot ? or Hot+cold ? The data we have received from investigation of the cosmic microwave background radiation by COBE satellite is very smooth and fast moving particles cannot clump together on this small scale from such a smooth initial clumping. Actually we need cold dark matter to get a better theory of dark matter and nowadays hot dark matter is jused a part of mixed dark matter theory.