Cold Dark Matter

Cold dark matter (CDM) is a central concept in modern cosmology, forming a foundational element of the standard ΛCDM model used to explain the evolution and structure of the universe. CDM refers to a hypothetical form of matter that interacts extremely weakly with electromagnetic radiation and ordinary baryonic matter, making it invisible to direct observation. Its designation as cold indicates that the particles composing it move at non-relativistic speeds in the early universe, enabling them to clump under gravity and seed the formation of cosmic structures.
In the ΛCDM framework, roughly 27 per cent of the universe’s total mass–energy content consists of dark matter, while dark energy accounts for about 68 per cent. Only a small fraction corresponds to familiar baryonic matter. CDM plays a crucial role in explaining how the initially smooth distribution of matter evolved into the complex network of galaxies, clusters and superclusters observed today.

Development of the CDM Theory

The cold dark matter hypothesis took formal shape in the early 1980s. Jim Peebles published pioneering work on CDM in 1982, while independent developments in warm dark matter theory were advanced by J. Richard Bond, Alex Szalay, Michael Turner, George Blumenthal, H. Pagels and Joel Primack. A major review article published in 1984 by Blumenthal, Sandra Moore Faber, Primack and Martin Rees provided a comprehensive theoretical synthesis, outlining how CDM could drive hierarchical structure formation.
According to this paradigm, small-scale density fluctuations in the early universe collapsed first, forming dwarf galaxies and smaller dark matter clumps. These structures merged repeatedly to form larger galaxies and galaxy clusters. The contrasting hot dark matter theory, influential only briefly, proposed a top-down process in which large sheets fragmented into galaxies, but this model became less favoured by the late twentieth century due to its inconsistency with observed large-scale structure.

Structure Formation and Observational Support

Observational evidence, such as measurements of the cosmic microwave background, galaxy distributions and gravitational lensing, broadly supports CDM as the primary driver of structure formation. Dwarf galaxies are particularly important in this framework, representing the earliest bound objects formed from small-scale density perturbations. They serve as potential building blocks for larger systems and provide a testing ground for cosmological models.
The ΛCDM model effectively explains the overall arrangement of matter in the universe and remains the dominant cosmological model.

Candidate Constituents of Cold Dark Matter

Because CDM interacts chiefly through gravity, its composition is unknown, although several categories of candidates have been proposed:

• Axions: Extremely light particles that could behave as CDM if produced in sufficient quantities. Axions have gained prominence since the late 2010s, partly because they address the strong CP problem in quantum chromodynamics. Despite theoretical appeal, they have not been detected. They belong to the broader category of WISPs, which refers to very weakly interacting sub-eV particles.
• MACHOs: Massive astrophysical compact halo objects such as black holes, neutron stars, white dwarfs, faint stars or non-luminous bodies. Searches employing gravitational lensing have placed strict limits on their abundance, leading most researchers to discount MACHOs as a major component of dark matter.
• WIMPs: Weakly interacting massive particles long considered prime candidates due to predictions from physics beyond the Standard Model. Direct detection experiments and accelerator searches have so far failed to observe WIMPs, diminishing their favour in recent years, though research continues.

Some experiments, including DAMA/NaI and DAMA/LIBRA, have reported signals interpreted as dark matter detection, but these results remain disputed due to the lack of corroboration from other experiments.

Challenges and Tensions in the CDM Framework

Despite its success on large scales, CDM faces several challenges, especially when compared with observations on galactic and sub-galactic scales:

• Cuspy halo problem: Simulations predict steep central density profiles in dark matter halos, but observed galaxy rotation curves suggest flatter cores.
• Dwarf galaxy problem: CDM models anticipate a far greater number of dwarf galaxies around systems like the Milky Way than currently observed.
• Satellite disk problem: Rather than being isotropically distributed, many dwarf galaxies in the Local Group orbit in thin, coherent planes, conflicting with CDM expectations.
• High-velocity galaxy problem: Members of the NGC 3109 association have velocities too large to fit comfortably within CDM predictions for local dynamics.
• Galaxy morphology problem: Hierarchical merging should produce classical bulges in massive galaxies, yet a high fraction of observed galaxies are bulgeless or exhibit pure-disc morphology. Studies indicate that this discrepancy persists across billions of years and may be difficult to reconcile within the CDM model.
• Fast galaxy bar problem: Bars in disc galaxies rotate more quickly than expected if they were embedded in massive CDM halos, which should slow them through dynamical friction.
• Small-scale crisis: Collectively refers to discrepancies such as overprediction of central dark matter concentration and dwarf galaxy abundance.
• High-redshift galaxy formation: Spectroscopic observations from the James Webb Space Telescope have confirmed extremely early, massive galaxies at redshifts above 13. Their rapid emergence challenges constraints imposed by ΛCDM, though adjustments to assumptions such as the stellar initial mass function may alleviate this tension.