• • • Dark matter is a form of that is thought to account for approximately 85% of the matter in the, and about a quarter of. The majority of dark matter is thought to be non- in nature, possibly being composed of some as-yet undiscovered.

Its presence is implied in a variety of observations, including effects that cannot be explained unless more matter is present than can be seen. For this reason, most experts think dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable, such as, and is thus invisible to the entire, making it extremely difficult to detect using usual astronomical equipment. The primary evidence for dark matter is that calculations show that many 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 unseen matter. Other lines of evidence include observations in, from the, from astronomical observations of the 's current structure, from the, from mass location during, and from the motion of galaxies within.

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In the standard model of cosmology, the total of the universe contains 5% and, 27% dark matter and 68% of an unknown form of energy known as. Thus, dark matter constitutes 85% of total, 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 matter and radiation, except through gravity. The primary candidate for dark matter is some new kind of that has, in particular, (WIMPs), or (GIMPs).

Many experiments to directly detect and study dark matter particles are being actively undertaken, but none has yet succeeded. Dark matter is classified as cold, warm, or hot according to its (more precisely, its ). Current models favor a scenario, in which by gradual accumulation of particles. Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists, intrigued by certain observations that do not fit the dark matter theory, argue for various modifications of the standard laws of, such as,,. These models attempt to account for all observations without invoking supplemental non-baryonic matter. See also: In standard cosmology, matter is anything whose energy density scales with the inverse cube of the, i.e., ρ ∝ a −3. This is in contrast to radiation, which scales as the inverse fourth power of the scale factor ρ ∝ a −4, and a, which is independent of a.

These scalings can be understood intuitively: for an ordinary particle in a cubical box, doubling the length of the sides of the box decreases the density (and hence energy density) by a factor of eight ( 2 3). For radiation, the decrease in energy density is larger because an increase in scale factor causes a proportional.

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A cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration. In principle, 'dark matter' means all components of the universe that are not visible but still obey ρ ∝ a −3. In practice, the term 'dark matter' is often used to mean only the non-baryonic component of dark matter, i.e., excluding '.' Context will usually indicate which meaning is intended. Observational evidence [ ]. Models of rotating disc galaxies in the present day (left) and ten billion years ago (right). In the present-day galaxy, dark matter—shown in red—is more concentrated near the center and it rotates more rapidly (effect exaggerated).

The arms of rotate around the galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it, similar to the. From, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System.