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, but forms by fragmentation, 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 as a description of how the universe went from a smooth initial state at early times 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.
Composition
Dark matter is detected through its gravitational interactions with ordinary matter and radiation. As such, it is very difficult to determine what the constituents of cold dark matter are. The candidates fall roughly into three categories:
Axions, very light particles with a specific type of self-interaction that makes them a suitable CDM candidate. Axions have the theoretical advantage that their existence solves the strong CP problem in quantum chromodynamics, but axion particles have only been theorized and never detected.
Massive compact halo objects, large, condensed objects such as black holes, neutron stars, white dwarfs, very faint stars, or non-luminous objects like planets. The search for these objects consists of using gravitational lensing to detect the effects of these objects on background galaxies. Most experts believe that the constraints from those searches rule out MACHOs as a viable dark matter candidate.
Weakly interacting massive particles. There is no currently known particle with the required properties, but many extensions of the standard model of particle physics predict such particles. The search for WIMPs involves attempts at direct detection by highly sensitive detectors, as well as attempts at production of WIMPs by particle accelerators. WIMPs are generally regarded as one of the most promising candidates for the composition of dark matter. The DAMA/NaI experiment and its successor DAMA/LIBRA have claimed to have directly detected dark matter particles passing through the Earth, but many scientists remain skeptical because no results from similar experiments seem compatible with the DAMA results.
Challenges
Several discrepancies between the predictions of the particle cold dark matter paradigm and observations of galaxies and their clustering have arisen: ; The cuspy halo problem: The density distributions of dark matter halos in cold dark matter simulations are much more peaked than what is observed in galaxies by investigating their rotation curves. ; The missing satellites problem: Cold dark matter simulations predict large numbers of small dark matter halos, more numerous than the number of small dwarf galaxies that are observed around galaxies like the Milky Way. ; The disk of satellites problem: Dwarf galaxies around the Milky Way and Andromeda galaxies are observed to be orbiting in thin, planar structures whereas the simulations predict that they should be distributed randomly about their parent galaxies. ; Galaxy morphology problem: If galaxies grew hierarchically, then massive galaxies required many mergers. Major mergers inevitably create a classical bulge. On the contrary, about 80% of observed galaxies give evidence of no such bulges, and giant pure-disc galaxies are commonplace. That bulgeless fraction was nearly constant for 8 billion years. Some of these problems have proposed solutions, but it remains unclear whether they can be solved without abandoning the CDM paradigm.
