Could you elaborate? It’s a set of observations about “something” that behaves like matter[1], and interacts only gravitationally. There are many theories prop

Could you elaborate? It’s a set of observations about “something” that behaves like matter[1], and interacts only gravitationally. There are many theories proposed but none has any real answers or solutions. The following is taken from Cern[2]: > Many theories say the dark matter particles would be light enough to be produced at the LHC. If they were created at the LHC, they would escape through the detectors unnoticed. However, they would carry away energy and momentum, so physicists could infer their existence from the amount of energy and momentum “missing” after a collision. Dark matter candidates arise frequently in theories that suggest physics beyond the Standard Model, such as supersymmetry and extra dimensions. One theory suggests the existence of a “Hidden Valley”, a parallel world made of dark matter having very little in common with matter we know. If one of these theories proved to be true, it could help scientists gain a better understanding of the composition of our universe and, in particular, how galaxies hold together. And thus at least whoever wrote this and the people who approved that publication agree with the notion that it is observations for which no concrete theory exists that explains them. [1] i used “behaves like matter” in the sense that follows this definition: > In standard cosmological calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, 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 cosmological constant, which does not change with respect to a (ρ ∝ a0). The different scaling factors for matter and radiation are a consequence of radiation redshift: For example, after gradually doubling the diameter of the observable Universe via cosmic expansion of General Relativity, the scale, a, has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled);[51] the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.[d] The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.[52][e] In principle, "dark matter" means all components of the universe which 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 "missing baryons". Context will usually indicate which meaning is intended. https://en.m.wikipedia.org/wiki/Dark_matter https://en.m.wikipedia.org/wiki/Dark_matter [2] https://home.cern/science/physics/dark-matter https://home.cern/science/physics/dark-matter