Once it was realized, decades ago, that there didn't appear to be enough mass in the Universe to explain things like the rotation of galaxies and the movements of star clusters, researchers began to ponder where all the missing mass had gone.
The conclusion was that either out understanding of physics, i.e. general relativity, was flawed, or perhaps the mass was still there, but simply not visible.
While it is still possible that we are missing something fundamental in our theories of gravity, the second option has been more palatable to physicists. And out of this revelation was born the idea of dark matter.
Cold Dark Matter (CDM)
Theories of dark matter can actually be slotted into three general groups: Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM).
Of the three, CDM has long been the leading candidate (though some still favor a combination theory, where aspects of all three types of dark matter exist together to make up the total missing mass).
Cold Dark Matter is thought manifest itself as some as-yet-undtected exotic particle, but one with very specific properties.
First, and perhaps most obviously, it would have to lack an interaction with the electromagnetic force. This is necessitated by the fact that dark matter is, well, dark, therefore it does not interact, reflect or radiate energy in the electromagnetic spectrum.
The candidate particle would, however, have to interact with the any gravitational field, as its purpose in our model is to explain certain gravitational interactions.
Of the two remaining fundamental forces, the strong and weak forces, there is some debate as to how any candidate matter would behave. If CDM interacted with the strong nuclear force - the force that binds nuclei - then the particles should be easier to detect. The fact that we have never measured such interactions indicates that dark matter does not interact in such ways.
However, at least some dark matter candidates, like neutrinos and neutrino-like particles (popular in Hot and Warm Dark Matter models) can have weak interactions. Other candidates, like Axions, would not necessarily interact with the weak force.
Candidate Cold Dark Matter Objects
While no known matter meets all of the criteria for Cold Dark Matter, there are at least three theoretical particles, thought to exist for a variety of reasons that could be forms of CDM should they turn out to exist.
- Weakly Interacting Massive Particles: Also known as WIMPs, these particles, but definition, meet all the needs of CDM. However, no such particle has ever been found to exist. WIMPs have become the catch all term for all Cold Dark Matter candidates, regardless of why the particle is thought to arise. However, there are a class of WIMPs that could exist out of non-thermal interactions, but such particles would actually be considered Warm Dark Matter (WDM).
- Axions: These particles were postulated to exist in order to explain a problem in Quantum Chromodynamics (QCD). As a happy coincidence Axions also posses (at least marginally) the necessary properties of dark matter. However, they may actually posses weak electromagnetic interactions and have a relatively small mass. So these seem less likely to be the answer.
- MACHOs: Don't you just love all the names physicists come up with? Anyway, Massive Compact Halo Objects (MACHOs) are objects like black holes, ancient neutron stars, brown dwarfs and planetary objects. These objects are all non-luminous and are massive. But because of their large size, both in terms of volume and mass, they would be relatively to detect by monitoring localized gravitational interactions. The observed motion of galaxies, for instance, is uniform in a way that would be hard to explain if these objects supplied the missing mass. Furthermore, star clusters would require a very uniform distribution of such objects, which seems very unlikely. Also, the sheer number of MACHOs that would be needed to explain the missing mass doesn't seem to be in agreement with Big Bang cosmology.
Ultimately, the mystery of dark matter doesn't seem to have an obvious answer. Even the best guesses at times seem like quite a stretch. In the meantime researchers continue to design experiments to search for these elusive particles and objects with the hopes that we can one day understand the nature of the Universe.
