In the cold dark matter theory, structure grows hierarchically, with small objects collapsing first and merging in a continuous hierarchy to form more and more massive objects. In the hot dark matter paradigm, popular in the early eighties, 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 strongly disagree with observations of large-scale structure, whereas the cold dark matter paradigm is in general agreement with the observations. Two important discrepancies between the predictions the cold dark matter paradigm and observations of galaxies and their clustering in space have arisen, however, creating a crisis for the cold dark matter picture; The cuspy halo problem, and the missing satellites problem.
The theory makes no predictions about exactly what the cold dark matter particles are, and the biggest weakness in cold dark matter is that there is no known particle that has the required properties. One candidate is a hypothetical particle known as a WIMP or Weakly Interacting Massive Particle.
See also: dark matter