Dark matter is hypothetical invisible matter in the universe, which does not emit or absorb light and other electromagnetic radiation (Hence, "dark" matter). Its existence is inferred because the galaxies and galaxy clusters should swirl apart in a few million years, unless there's something odd with gravitation, or there's a mass that we can't see.
Measurements of galactic rotations and galaxy cluster dynamics, along with other evidence, have led scientists to infer that there must be much more mass in the universe than we can currently detect. It is this hypothetical undetected mass that has been named "dark matter."
Visible matter — that is, the entirety of the stars, galaxies, planets, etc. that we can see — is thought to comprise only 4 percent of the universe's energy. Roughly 23 percent is dark matter, and the remaining 73 percent is thought to be dark matter's even more ominous cousin dark energy.
Trouble-making skeptics have on occasion pointed out that science is supposed to modify theories to fit observation, and as such, rather than hypothesize about undetectable dark entities which we cannot measure, it might be prudent to check there's nothing wrong with the theory of gravitation. Thus, the scientific alternatives to the dark matter theory are modified gravitation theories, the most renowned of which is the MOND (Modified Newtonian dynamics) of Mordehai Milgrom, and some similar theories, for example TeVeS. These modified gravity hypotheses are superior at predicting the rotation curves of galaxies, but they have a rather difficult time accounting for anomalies such as the Bullet Cluster and the vast excess of mass in galaxy superclusters. Dark matter is also necessary to account for certain features of the CMB power spectrum, and must be included in models of the development of the universe to bring them into accord with observations. Dark Matter does seem to be a rather inelegant "patch" to cosmology, and attempts to directly detect it have failed, but so far, it is the only hypothesis which adequately explains the observed cosmological and galactic data.
Not to be confused with: phlogiston.
The proposals for what this dark matter is are subdivided into MACHOs, WIMPs and axions, very vaguely indicating (for persons with a gymnastic imagination) what is referred to:
- MACHOs, Massive astrophysical compact halo objects, are heavy compact objects that does a great deal of noise when colliding with something, which they seldom do because of their diminutive size,
- WIMPs, Weakly Interactive Massive Particles, are exotic fairy particles of fat constitution, that are too shy to be seen and to interact with matter that we see in daily life,
- axions, branded after an extinct detergent, cleans away the other two, and instead presumes a very low mass particle existing in large numbers.
WIMPs are truly exotic elementary particles of kinds that we don't know ... yet ... for sure, what it is. Some of the more important candidates are:
- micro black holes, a.k.a. holeums,
- very high-energy neutrinos,
- neutralinos, a particle expected to exist if supersymmetry is a viable replacement for the standard model,
- sterile neutrinos, one or a few particle types expected to exist if the see-saw mechanism is viable as a replacement for the standard model,
- neutral electron, a particle inferred (beside two kinds of neutrinos) by the not very popular or well known Heim theory, mostly because it uses too complicated mathematics.
The standard model of current quantum physics crashed when the solar neutrino problem was solved. The solar neutrino problem occurred when first the neutrino emission of Sun was measured up to only a third of what stellar fusion models and standard model physics predict. It took decades till the problem was solved: the Sun really did produce enough neutrinos to defend stellar fusion models, but the neutrinos morphed themself back and forth between the three known kinds of neutrinos: electron, muon and tauon neutrinos. That implies that neutrinos have mass, which they don't, according to the standard model.
Various replacements for the standard model are prepared for test: the supersymmetry models, the seesaw mechanism fix on the standard model and various quantum gravity models. Many particle physics experiments are currently running in order to replace the standard model collapse wit a better model, to solve the dark matter problem and other tough physics problems, such as why do we have mass (except eating too much junk food, that is).
 Science woo
Science woo pushers tend to seize on the mysteries surrounding dark matter and dark energy, and may claim that either is a manifestation of whatever nonsense they distribute. This pretension of being scientific when being not, is the characteristic signature of pseudoscience — it is profitable for the charlatan until sharp eyes lets the sharp tongue slash the humbug.
 See also
- Patricia Burchat sheds light on dark matter (Video)
- Dark matter comes out of the cold
- Most of Our Universe is Missing
- Dark Matter - introduction