Dark matter is classified as "cold", "warm", or "hot" according to its velocity (more precisely, its free streaming length). Many experiments to detect and study dark matter particles directly are being actively undertaken, but none have yet succeeded. The other main possibility is that dark matter is composed of primordial black holes.
The leading explanation is that dark matter is some as-yet-undiscovered subatomic particle, such as weakly interacting massive particles (WIMPs) or axions. It is not known to interact with ordinary baryonic matter and radiation except through gravity, making it difficult to detect in the laboratory. ĭark matter's exact identity remains a mystery. 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 mass–energy content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as dark energy. Evidence for dark matter comes from many different angles, such as galaxy dynamics and formation, 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. Its existence is implied by various astrophysical observations which cannot be explained by general relativity unless more matter is present than can be seen. It 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 and is, therefore, difficult to detect. Dark matter is a hypothetical form of matter thought to be the predominant type of matter in the universe.
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