Cold dark matter (or CDM) is a refinement of the big bang theory, as well as being one possible variation of the more generic Dark Matter theory. According to the Big Bang theory, the universe emerged from an extremely dense and hot state (singularity). ... In astrophysics and cosmology, dark matter is matter, not directly observed and of unknown composition, that does not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter. ...

In the cold dark matter theory, structure grows hierarchically, with small objects collapsing first and merging in a continuous hierarchy to form more and more massive objects. In the hot dark matter paradigm, popular in the early eighties, structure does not form hierarchically (bottom-up), but rather forms by fragmentation (top-down), with the largest superclusters forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way. Hot dark matter is a form of dark matter, which consists of particles that travel with relativistic velocities. ... Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. ... It has been suggested that Andromeda-Milky Way collision be merged into this article or section. ...

Cold Dark Matter theory makes no predictions about exactly what the cold dark matter particles are but there are two main candidates

• WIMPs or Weakly Interacting Massive Particles assumes that the dark matter is some sort of heavy unknown particle. Unfortunately, there is no known particle with the required properties. The particles that have little interaction with normal matter, such as neutrinos, are all very light, and hence would be fast moving or hot. WIMP-like particles are predicted by supersymmetry, although none of the large number of new particles in supersymmetry has been observed.

• MACHOs or Massive Compact Halo Objects assumes that the gravitational effects that apparently indicate the presence of Dark Matter, are caused by regular baryonic matter which emits little or no radiation and drifts through interstellar space unassociated with any solar system, such as a wandering blackhole or neutron star.

This bottom up model of structure formation (of which Cold Dark Matter plays a necessary part) is consistent with the structures observed in the universe through galaxy surveys. Large computer simulations of billions of dark matter particles have been used to show this. In astrophysics, WIMPs, or weakly interacting massive particles, are hypothetical particles serving as one possible solution to the dark matter problem. ... This article or section is in need of attention from an expert on the subject. ... Look up Macho in Wiktionary, the free dictionary This is a disambiguation page: a list of articles associated with the same title. ...

However, there are some problems still unresolved. One example of this, is the dwarf galaxy problem. The dwarf galaxy problem is one that arises from numerical cosmological simulations that predict the evolution of the distribution of matter in the universe. ...

It is unclear as to how intractable these problems are; whether they represent a crisis or simply a nuisance is a matter of some dispute in the cosmological community.

See also

• Dark matter
  • Hot dark matter (HDM)
  • Warm dark matter (WDM)