Within the expansion model, cold dark matter has the same equation of state as normal matter, although it has some fundamentally different properties. However, the relevance of the previous statement is that the importance of cold dark matter also lies within the matter-dominated era of the universe.
The existence of cold dark matter (CDM) was initially very speculative, and possibly remains so. This existence was based on the observation of a number of gravitational anomalies, e.g. rotation anomalies in spiral galaxies through to gravitational lensing around unseen objects. However, models of the early formation of structure within the universe were then seen as strengthening the support for this idea. Basically, it appears that without dark matter, the process of `structural coagulation` could not have progress in a manner that would have led to the current observed universe. What added to the validity of this idea was that the estimated energy density (23%) was originally based only on the amount of dark matter required to address these gravitational anomalies. Subsequent computer simulations then showed that this amount of cold dark matter would also support the process of clumping and the formation of cobweb-like strands within the voids, which were only later shown to align with observations.
- MAssive Compact Halo Objects (MACHOs):
The first hypothesis referred to dark matter in the form of MAssive Compact Halo Objects (MACHOs) consisted of Jupiter-sized planets, brown dwarf stars, faint low-mass stars, white dwarf stars and even black holes. However, this idea could only account for a fraction of the dark matter required to explain a growing list of anomalies.
- Weakly Interacting Massive Particles (WIMPs):
The second hypothesis is known as Weakly Interacting Massive Particles (WIMPs) was then forwarded, which is now also known as Cold Dark Matter. While this hypothesis underpins the current concordance model, it should be pointed out that the existence of WIMPs has not yet been verified. However, WIMPs are described in terms of particles that may only interact through the weak nuclear force and gravity. On the basis that there is no electromagnetic interaction, WIMPs cannot be seen directly; equally if there is little to no strong nuclear force, there will be little to no interaction with atomic nuclei. Therefore, WIMPs are often described in terms similar to neutrinos, although they would have to be more massive and hence slower.
As such, it would seem that WIMPs have got the better of MACHOs and point to some level of poetic justice within the universe, but then again, probably not. As indicated above, the energy density of cold dark matter is modelled on the same equation of state as baryon matter, i.e. [ω=0], and therefore its energy density changes as an inverse of volume and therefore is also assumed to have no associated pressure [P].