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Dark matter is a hypothesis that there is a large amount of mass in the Universe (most of it, even) which cannot be detected using light and other electromagnetic radiation. The term "dark" in its name refers not to this property, however, but the fact that we simply do not know its properties, besides that it has mass. 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 (the latter being dark matter).
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 on average only 4.9% of the Universe's energy. Roughly 26.8% is dark matter, and the remaining 68% is thought to be dark matter's even more ominous cousin dark energy. Visible matter then is about 15% on average of all matter (4.9%/(4.9%+26.8%)). There have been discovered, however, dark galaxies with few stars that contain only about 2% of visible matter..
There are three hypothetical types of dark matter: "hot", whose constituent particles move at almost the speed of light, "cold", whose particles are much slower, and "warm", which is intermediate. Note that this does not preclude "mixed" models, where dark matter is composed of a mixture of two or even the three types. "Cold" dark matter is the one usually referred to by the shorthand.
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 that 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 just as good at predicting the rotation curves of most 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, and all other attempts to explain the data end up even less elegant.
The properties of a possible faint signal coming from the dawn of the Universe, when the first stars formed 180 million years after the Big Bang, have been interpreted as being caused by dark matter, more exactly particles of it less than five times as massive as a hydrogen atom. However other more mundane alternatives may be the actual reason
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 make 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 massive 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,
- 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.
Neutrinos are already WIMPs, but are too "hot" to account for the way dark matter has been seen to act in e.g. the Bullet cluster; a proper dark matter candidate must be massive enough not to go nearly the speed of light with even a relatively small momentum.
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 the 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 themselves 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 with a better model, to solve the dark matter problem and other tough physics problems; some tough questions, "such as why do we have mass (except eating too much junk food, that is)?", have been answered, by the discovery of the higgs boson, confirming the existence of the higgs mechanism that gives many elementary particles their mass (though most of the mass of baryonic beings such as all life comes from the strong binding energy of nucleons--protons and neutrons).
There have been multiple times in the past where the movement of astronomical objects differed from the predictions of then current gravitational theory. In the 1820s, French astronomer Alexis Bouvard observed that the movement of Uranus was substantially different from predictions of Newtonian gravity. Assuming that some unseen ("dark") planet was perturbing Uranus' orbit, John Couch Adams and Urbain Le Verrier made predictions about the position of a new planet. Twenty years after Uranus' weird motion was discovered, on September 23, 1846, a new planet was discovered by German astronomer Johann Gottfried Galle nearer the position predicted by Verrier. The dark matter was subsequently named Neptune.
In the twentieth century, observations of the orbit of Mercury around the Sun were shown to deviate significantly from Newtonian predictions. Following the previous example of the discovery of Neptune, it was predicted that a new planet even closer to the Sun would be found that pulled on Mercury just the right way to explain its motion. This planet was named Vulcan in anticipation of its discovery. No such planet was ever found. What finally explained the orbit of Mercury was a new theory of gravity: Einstein's General Relativity.
Both dark matter and modified gravity have been solutions to astronomical observations of weird motion.
In more Earthbound physics, nuclear decays were found to not conserve momentum and energy. One idea was that conservation laws only approximately held and some new, deeper theory would be required to explain measurements. Another idea was that a ghostly, mysterious particle was escaping the experiment unmeasured, carrying away momentum and energy. Years later, we now call the mysterious particle the neutrino.
Until a dark matter particle or substance is identified or a better theory of gravity is discovered, the correct action is to keep collecting evidence.
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 let the sharp tongue slash the humbug.
Biologists borrowed the term "dark matter" from physicists to refer to microbes (generally bacteria and archaea) that they are unable to culture in the laboratory and therefore unable to analyze. It is suspected that the microbial dark matter vastly outnumbers the known microbes.
- Patricia Burchat sheds light on dark matter (Video)
- Dark matter comes out of the cold
- Most of Our Universe is Missing
- Dark Matter - introduction
- How Dark Matter Works
- Star-starved galaxies fill the cosmos: As researchers find hundreds of galaxies with scarce starlight, questions pile up by Christopher Crockett (11:00am, November 29, 2016) Science News.
- Adrian Cho. "Signal from age of the first stars could shake up search for dark matter." 2018.
- Chris Miller. "Cosmic Hide and Seek: the Search for the Missing Mass." 1995.
- See, for instance, this, which claims that dark matter and dark energy are obviously due to some life force called "Zero Energy."
- See the Wikipedia article on Microbial dark matter.