Black Matter

Dark matter or Black Matter is a type of matter which has not yet been directly observed, but is thought to form a fundamental part of the universe. Very strong evidence suggests its existence, and a broad scientific consensus agrees that some form of dark matter is widely present in the universe and has shaped the universe's structure. Dark matter plays a central role in the current understanding of the universe.

It is suspected that the reason this "dark" matter has not yet been directly observed is because, apart from gravity, it almost never interacts in ways that can be detected, and because it comprises new kinds of elementary particles that have not yet been discovered - meaning it is distinct from "ordinary" matter such as protons, neutrons, electrons, and neutrinos. The name "dark matter" refers to its elusive nature, and also to the fact that it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible (or 'dark') to the entire electromagnetic spectrum, making it extremely difficult to detect using usual astronomical equipment. At present, its indirect detection and study is limited to observations by its gravitational interaction with "ordinary" matter. Its effects on ordinary matter allow us to infer its presence and some of its properties. Many different lines of evidence agree on the extent of dark matter in the observable universe.

The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of matter beyond that which can be observed. The conclusion is that the universe contains far more matter in a form that cannot be currently detected. The 'envelope' of dark matter which is estimated to dominate galactic mass is known as a dark matter halo; the Milky Way galaxy's halo is estimated to be around 1.5 million light years across - nearly 15 times the size of the visible galaxy. Dark matter's properties are inferred from observations in gravitational lensing, from the cosmic microwave background which shows the structure of the universe early in its history, from astronomical observations of the observable universe's current structure, and from evidence about the formation and evolution of galaxies, from mass location during galactic collisions, and from the motion of stars within galaxies, and of galaxies within galactic clusters. Its existence would also explain a number of otherwise puzzling astronomical observations. Many experiments to directly detect and study dark matter particles are being actively undertaken, however none have yet succeeded. It is thought that when dark matter is directly observed, it will be composed of weakly interacting massive particles ("WIMPs") that only interact through gravity and the weak force. Dark matter is classified as "cold", "warm", "or "hot" according to its velocity (more accurately, its free streaming length). Most scenarios involving familiar objects such as interstellar dust, large black holes, asteroids and other non-luminous or dense objects are thought to be ruled out by observations or the little present knowledge of dark matter's properties. Current models prefer a cold dark matter scenario, in which structures emerge by gradual accumulation of particles.

In the most widely accepted model of the universe, the standard model of cosmology (known as the Lambda-CDM model), the total mass–energy of the universe contains 4.9% ordinary matter and energy, 26.8% dark matter and 68.3% of an unknown form of energy known as dark energy. Thus, dark matter constitutes 84.5% of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content. The great majority of ordinary matter in the universe is also unseen. Visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass-energy density of the universe. The Bolshoi Cosmological Simulation, a supercomputer simulation of the universe, shows that in a universe of the kind suggested by present theories, dark matter would form huge strands (filaments) and voids, and "ordinary" matter would be attracted to these by gravity, giving a structure to the universe very similar to what can be seen today.

Although the existence of dark matter is generally accepted by most of the astronomical community, a minority of astronomers,  motivated by the lack of conclusive identification of dark matter, or by observations that don't fit the model, argue for various modifications of the standard laws of general relativity, such as MOND, TeVeS, and conformal gravity that attempt to account for the observations without invoking additional matter.