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In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation.Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed.
As "dark matter", baryonic dark matter is undetectable by its emitted radiation, but its presence can be inferred from gravitational effects on visible matter. This form of dark matter is composed of "baryons", heavy subatomic particles such as protons and neutrons and combinations of these, including non-emitting ordinary atoms.
Unlike dark matter, ordinary matter can lose energy by many routes, which means that as it collapses, it can lose the energy which would otherwise hold it apart, and collapse more quickly, and into denser forms. Ordinary matter gathers where dark matter is denser, and in those places it collapses into clouds of mainly hydrogen gas.
Dark matter is called ‘dark’ because it’s invisible to us and does not measurably interact with anything other than gravity. It could be interspersed between the atoms that make up the Earth ...
Based on the 2013 data, the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. On 5 February 2015, new data was released by the Planck mission, according to which the age of the universe is 13.799 ± 0.021 billion years old and the Hubble constant was measured to be 67.74 ± 0.46 (km/s)/Mpc .
The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved, but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).
The presence of dark matter (DM) in the halo is inferred from its gravitational effect on a spiral galaxy's rotation curve.Without large amounts of mass throughout the (roughly spherical) halo, the rotational velocity of the galaxy would decrease at large distances from the galactic center, just as the orbital speeds of the outer planets decrease with distance from the Sun.
In cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. According to the current standard model of cosmology, Lambda-CDM model , approximately 27% of the universe is dark matter and 68% is dark energy , with only a small fraction being the ordinary baryonic matter that composes stars , planets , and living organisms.