Dark matter is classified as "cold," "warm," or "hot" according to its velocity (more precisely, its free streaming length). Many experiments to directly detect and study dark matter particles are being actively undertaken, but none have yet succeeded. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, particularly weakly interacting massive particles (WIMPs), though axions have drawn renewed attention due to the non-detection of WIMPs in experiments. Most dark matter is thought to be non-baryonic it may be composed of some as-yet-undiscovered subatomic particles. īecause no one has directly observed dark matter yet – assuming it exists – it must barely interact with ordinary baryonic matter and radiation except through gravity. Thus, dark matter constitutes 85% of the total mass, while dark energy and dark matter constitute 95% of the total mass-energy content. In the standard Lambda-CDM model of cosmology, the total mass-energy content of the universe contains 5% ordinary matter and energy, 27% dark matter, and 68% of a form of energy known as dark energy.
Other lines of evidence include observations in gravitational lensing and 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. Some galaxies would not have formed at all and others would not move as they currently do. The primary evidence for dark matter comes from calculations showing that many galaxies would behave quite differently if they did not contain a large amount of unseen matter. For this reason, most experts think that dark matter is abundant in the universe and has had a strong influence on its structure and evolution. Various astrophysical observations - including gravitational effects which cannot be explained by currently accepted theories of gravity unless more matter is present than can be seen - imply dark matter's presence. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect, or emit electromagnetic radiation (like light) and is, therefore, difficult to detect. Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe.
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