The first time that dark matter was suggested to exist, it probably sounded like a cop out.
In the early part of the 20th century, physicists were having a difficult time explaining the rotation curves of other galaxies. In other words, galaxies didn't appear to have enough mass to explain their observed rotation.
Then in 1933 physicist Fritz Zwicky proposed that perhaps the mass was there, but was non-luminous (not visible to the naked eye).
Subsequent studies on everything from galactic rotation curves, gravitational lensing, star cluster movements and measurements of the cosmic microwave background all indicate the presence of more mass than what we can directly measure in the electromagnetic spectrum.
In other words, in just about every way that we measure the Universe there appears to be more there than what we can see. So what is this "dark matter"?
Dark "Normal" Matter
Normal, luminous matter is made up of baryons - particles such as protons and neutrons. At first, dark matter was believed to also be made up of such material, but simply emitted little to no electromagnetic radiation.
While it is likely that at least some dark matter is composed of baryonic dark matter, it is likely only a small part of all dark matter.
Observations of the cosmic microwave background coupled with our understanding of the big bang theory, lead physicists to believe that only a small amount of baryonic matter would continue to survive today that is not incorporated in a solar system or stellar remnant.
Non-Baryonic Dark Matter
It seems unlikely that the missing matter of the Universe is to be found in the form of normal, baryonic matter. Therefore, researchers believe that a more exotic particle is likely to provide the missing mass.
Exactly what this matter is, and how it came to be is still a mystery. However physicists have identified the three most likely types of dark matter and the candidate particles associated with each type.
- Cold Dark Matter (CDM): The most likely candidate for dark matter is Cold Dark Matter (CDM). However, there isn't a strong candidate particle known to exist. The leading candidate for CDM is known as a Weakly Interacting Massive Particle (WIMP). However, there is a general lack of justification for existence of such particles; namely we are not certain how they were arise under natural circumstance. To investigate, researchers are conducting particle physics experiments hopping that collisions would produce a candidate particle. Other possibilities for CDM include Axions - theoretical particles needed to explain certain phenomenon in Quantum Chromodynamics (QCD). Though these particles also have never been detected. And, finally, MACHOs (MAssive Compact Halo Objects) could explain the mass, but the specific dynamics remain a reach. These objects would include black holes, ancient neutron stars and planetary objects which are all non-luminous (or nearly so) and contain a significant amount of mass. The problem here is that there would have to be a lot of them (more than would be expected given the age of certain galaxies) and their distribution would have to be surprisingly (impossibly?) uniform.
- Warm Dark Matter (WDM): This form of dark matter is thought to be composed of sterile neutrinos. These are particles that are similar to normal neutrinos saving for the fact that they are much more massive and do not interact with the weak force. Another candidate for WDM is the gravitino. This is a theoretical particle that would exist should the theory of Supergravity - a blending of general relativity and supersymmetry - gain traction. Certainly evidence for the existence of a gravitino would be significant for both realms of physics.
- Hot Dark Matter (HDM): The subset of particles considered to be Hot Dark Matter are the only ones really known to exist: Neutrinos. The problem with this explanation is that neutrinos travel at nearly the speed of light and therefore would not "clump" together in ways that we project dark matter to. Also given that the neutrino is nearly massless an incredible amount of them would be needed to meet the needed deficit. One explanation is that there is a yet-undetected type or flavor of neutrino that would be similar to those already known to exist except would have a significantly larger mass (and hence perhaps slower speed).
In conclusion the best candidate for dark matter appears to be Cold Dark Matter, and specifically WIMPs. However there is the least justification and evidence for such particles (except for the fact that we can infer the presence of some form of dark matter). So we are a long way from having an answer on this front.
Alternative Theories to Dark Matter
Some have proposed that dark matter is actually just normal matter that is entrenched in supermassive black holes that are orders of magnitude greater in mass than those at the center of active galaxies. (Though some could also consider these objects Cold Dark Matter).
While this would help explain some of the gravitational perturbations observed in galaxies and galaxy clusters, they would not solve most of the galactic rotation curves.
Another, but less accepted theory, is that perhaps our understanding of gravitational interactions is wrong.
We base our expected values on general relativity, but it could be that there is a fundamental flaw in this approach and perhaps a different underlying theory describes large scale galactic rotation.
This again seems unlikely as tests of general relativity agree with the predicted values. Others are quick to point out, however, that these are all local measurements and perhaps the physical laws that govern large scale interactions are different depending on your location in the Universe.
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