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Wikipedia dicit:

Dark matter is hypothesized to be a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total mass–energy density or about 2.241×10−27 kg/m3. Support for its presence is drawn from a variety of astrophysical observations, including gravitational effects that under current theories of gravity do not make sense, unless more matter is present than can be seen. For this reason, the hypothesis has been created that dark matter exists, is abundant in the universe, and has had a strong influence on its structure and evolution. The name is due to the fact that by all observations, should dark matter exist, it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect or emit electromagnetic radiation, and is therefore difficult to detect.

Primary support for dark matter comes from calculations showing that many galaxies would fly apart, or that they would not have formed or would not move as they do, if they did not contain a large amount of unseen matter. Other lines of evidence include observations in gravitational lensing and in the cosmic microwave background, along with astronomical observations of the observable universe’s current structure, the formation and evolution of galaxies, mass location during galactic collisions, and the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of a form of energy known as dark energy. Thus, dark matter constitutes 85% of total mass, while dark energy plus dark matter constitute 95% of total mass–energy content.

Because dark matter has not yet been observed directly, if it exists, it must barely interact with ordinary baryonic matter and radiation, except through gravity. Most dark matter is thought to be non-baryonic in nature; it may be composed of some as-yet undiscovered subatomic particles. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular, weakly interacting massive particles (WIMPs). Many experiments to directly detect and study dark matter particles are being actively undertaken, but none have yet succeeded. Dark matter is classified as “cold”, “warm”, or “hot” according to its velocity (more precisely, its free streaming length). Current models favor a cold dark matter scenario, in which structures emerge by gradual accumulation of particles.

Video credit: NASA’s Goddard Space Flight Center/Paul Morris (USRA): Lead Producer/Cassandra Morris: Voice over Talent/Visualizations and Additional Footage: ESA/Hubble — Gravitational Lensing Animation/ESA/Hubble — Gravitational Lensing Simplified Visualization/R. Wesson/ESO — Very Large Telescope Footage