In the standard model of cosmology, the total mass–energy of the universe is said to contain 5% of ordinary matter and energy, 27% of dark matter, and 68% of what is known as dark energy. This article will only touch the surface of what dark matter really is, as it is a complex topic that can change over time.

The first scientist to talk about the idea of dark matter was Lord Kelvin in an 1884 talk in which he estimated the number of dark bodies in the Milky Way using the observed velocity dispersion of the stars orbiting around the center of the galaxy. He was able to use these measurements to estimate the mass of the galaxy, which was noticeable because he determined it was different than the mass of visible stars. This led Lord Kelvin to conclude that “many of our stars, perhaps a great majority of them, may be dark bodies.” 

In his 1906 paper titled, “The Milky Way and Theory of Gases”, Henri Poincaré became the first person to utilize a term similar to “dark matter”. The mention was in regards to a discussion of the work of Kelvin in regards to its feasibility. As Mr. Poincaré wrote the paper in French, the term dark matter was written as “matière obscure” in French.

One of the first scientists to hint at the dark matter existence was Dutch astronomer Jacobus Kapteyn in 1922. In 1932 another Dutch scientist, Jan Oort hypothesized the existence of dark matter when studying stellar motions in the local galactic neighborhood and found the mass in the galactic plane must be greater than what was observed.

In 1933, Swiss astrophysicist Fritz Zwicky made a similar inference when applying the virial theorem to the Coma Cluster and obtained evidence of unseen mass he called dunkle Materie (‘dark matter’). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. What was seen was that it was estimated to have a mass about 400 times more than was visually observable. Based upon these conclusions, Zwicky was able to infer that some unseen matter provided the mass and associated gravitation attraction to hold the cluster together. But these estimates were in error of by an order of magnitude, mainly because of an obsolete value of the Hubble constant.

Observational evidence

• Galaxy Rotation Curves
• Velocity Dispersions
• Galaxy Clusters
• Gravitational Lensing
• Cosmic Microwave Background
• Structure Formation
• Bullet Cluster
• Type Ia Supernova Distance Measurements
• Sky Surveys and Baryon Acoustic Oscillations
• Redshift-Space Distortions
• Lyman-Alpha Forest

Classification

The classification of dark matter can be divided into three categories: cold, warm, and hot, which has nothing to do with the actual temperature of the matter. Rather than referring to the actual temperature, classification and categorized is based upon velocity which, indicates how far the corresponding objects have moved due to random motions in the early universe. These movements in the early universe have occurred before the object movement slowed due to cosmic expansion, whose distance is called the free streaming length, abbreviated as FSL.

It is important to note that the following categories are setup with respect to the size of a protogalaxy. A protogalaxy is an object that will later evolve into a white dwarf, but that is another topic. Dark matter particles are classified as cold, warm, or hot according to their FSL, with the much smaller matter is classified as cold, similar sized matter is classified as warm, or much larger sized matter is considered hot.

• Cold Dark Matter offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy. This is the focus for dark matter research, as hot dark matter does not seem capable of supporting galaxy or galaxy cluster formation, and most particle candidates slowed early.
• Warm Dark Matter comprises particles with an FSL comparable to the size of a protogalaxy. Predictions based on warm dark matter are similar to those for cold matter on large scales, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies.
• Hot Dark Matter consists of particles whose FSL is much larger than the size of a protogalaxy. Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies the first objects that can form are huge supercluster-size pancakes, which then fragment into galaxies. Deep-field observations have shown that what more probably formed first was galaxies, which were followed by clusters and superclusters as galaxies clumped together.

Dark Matter Particle Detection

• Direct Detection Experiments have the aim of observing low-energy recoils of nuclei induced by interactions with particles of dark matter. These low energy-recoils should theoretically be passing through the Earth. When the nucleus passes through the sensitive detection devices, the energy recoil will be emitted in the form of light scintillation or phonons. Effective utilization of this direct detection devices require maintaining a low background, which means that many of these experiments operate deep underground to reduce the interference from cosmic rays.
• Indirect Detection Experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. Examples of indirect detection experiments include the Energetic Gamma Ray Experiment Telescope and the Fermi Gamma-Ray Space Telescope 
• Collider Searches allows scientists to utilize a collider in a laboratory to create and then detect dark matter particles. It has been hypothesized that scientists could perform experiments with the Large Hadron Collider in such a way that could allow for the detection of dark matter particles. Detecting dark matter using the LHC is going to take some time.

References And Further Reading

• Curlie article on subject
• Encyclopædia Britannica Entry
• A February 2017 Ars Technica article on the subject