Cold dark matter

In cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. Observations indicate that approximately 85% of the matter in the universe is dark matter, with only a small fraction being the ordinary baryonic matter that composes stars, planets, and living organisms. Cold refers to the fact that the dark matter moves slowly compared to the speed of light, while dark indicates that it interacts very weakly with ordinary matter and electromagnetic radiation. In cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. Observations indicate that approximately 85% of the matter in the universe is dark matter, with only a small fraction being the ordinary baryonic matter that composes stars, planets, and living organisms. Cold refers to the fact that the dark matter moves slowly compared to the speed of light, while dark indicates that it interacts very weakly with ordinary matter and electromagnetic radiation. The physical nature of CDM is currently unknown, and there are a wide variety of possibilities. Among them are a new type of weakly interacting massive particle, primordial black holes, and axions. The theory of cold dark matter was originally published in 1982 by three independent groups of cosmologists: James Peebles; J. Richard Bond, Alex Szalay, and Michael Turner; and George Blumenthal, H. Pagels, and Joel Primack.A review article in 1984 by Blumenthal, Sandra Moore Faber, Primack, and Martin Rees developed the details of the theory. In the cold dark matter theory, structure grows hierarchically, with small objects collapsing under their self-gravity first and merging in a continuous hierarchy to form larger and more massive objects. Predictions of the cold dark matter paradigm are in general agreement with observations of cosmological large-scale structure. In the hot dark matter paradigm, popular in the early 1980s and less so now, structure does not form hierarchically (bottom-up), but rather forms by fragmentation (top-down), with the largest superclusters forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way. Since the late 1980s or 1990s, most cosmologists favor the cold dark matter theory (specifically the modern Lambda-CDM model) as a description of how the universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation) to the lumpy distribution of galaxies and their clusters we see today—the large-scale structure of the universe. Dwarf galaxies are crucial to this theory, having been created by small-scale density fluctuations in the early universe; they have now become natural building blocks that form larger structures.