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Facts about Dark Matter for Kids

Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level. Instead, its existence and properties are inferred from its gravitational effects on visible matter, radiation, and the large scale structure of the universe.

  • Dark matter came to the attention of astrophysicists due to discrepancies between the mass of large astronomical objects determined from their gravitational effects, and the mass calculated from the “luminous matter” they contain; such as stars, gas and dust.
  • It was first postulated by Jan Oort in 1932 to account for the orbital velocities of stars in the Milky Way and Fritz Zwicky in 1933 to account for evidence of “missing mass” in the orbital velocities of galaxies in clusters.
  • Subsequently, other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies.
  • The most widely accepted explanation for these phenomena is that dark matter exists and that it is most likely composed of heavy particles that interact only through gravity and possibly the weak force; however, alternate explanations have been proposed, and there is not yet sufficient experimental evidence to determine which is correct.
  • According to observations of structures larger than solar systems, as well as Big Bang cosmology interpreted under the Friedmann equations and the FLRW metric, dark matter accounts for 23% of the mass-energy content of the observable universe.
  • In comparison, ordinary matter accounts for only 4.6% of the mass-energy content of the observable universe, with the remainder being attributable to dark energy.
  • Dark matter plays a central role in state-of-the-art modeling of structure formation and galaxy evolution, and has measurable effects on the anisotropies observed in the cosmic microwave background.
  • Study of nucleosynthesis in the Big Bang produces an upper bound on the amount of baryonic matter in the universe, which indicates that the vast majority of dark matter in the universe cannot be baryons, and thus does not form atoms.
  • There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some combination of these is also possible.
  • While sometimes appearing with lower mass-to-light ratios, measurements of ellipticals still indicate a relatively high dark matter content.
  • The most commonly proposed particles then became WIMPs (Weakly Interacting Massive Particles, including neutralinos), or axions, or sterile neutrinos, though many other possible candidates have been proposed.
  • The difference between modified gravity laws and quantum gravity laws is that modified gravity laws simply propose alternative behaviour of gravity at astrophysical and cosmological scales, without any regard to the quantum scale.
  • One group of alternative theories to dark matter assumes that the observed inconsistencies are due to an incomplete understanding of gravitation rather than to the presence of invisible matter.