Cold Dark Matter

Cold dark matter (or CDM) is a hypothetical form of matter that interacts very weakly with electromagnetic radiation (dark) and most of whose particles move slowly compared to the speed of light (cold). It is believed that approximately 80% of matter in the Universe is dark matter, with only a small fraction being the ordinary "baryonic" matter that composes stars and planets. As of 2006, most cosmologists favor the cold dark matter theory 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. The theory sees the role dwarf galaxies played crucial, as they are thought to be natural building blocks that form larger structures, created by small-scale density fluctuations in the early Universe. The theory was originally published in 1984 by United States physicists Joel R. Primack, George Blumenthal, and Sandra Moore Faber with UK scientist Martin Rees.

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 more and more larger and more massive objects. In the hot dark matter paradigm, popular in the early 1980s, 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. The predictions of hot dark matter disagree with observations of large-scale structures, whereas the cold dark matter paradigm is in general agreement with the observations.