Cold dark matter (CDM) is a hypothetical form of dark matter and is an element of the Lambda-CDM model. Dark matter is theorized to explain the dynamics of galaxies and galaxy clusters which appear to have more gravity than would be explained by the visible stars and gas.
Theorized dark matter can be classified as cold, warm, or hot, depending upon how far the theorized particles would travel unimpeded in the early universe. Cold dark matter is dark matter whose travel distance is less than the diameter of a protogalaxy. Warm dark matter and hot dark matter are the alternate types of theory.