Dark Matter

A hypothetical form of matter that does not emit, absorb, or reflect electromagnetic radiation — detectable only through its gravitational effects. Makes up approximately 27% of the total mass-energy of the observable universe, compared to 5% for ordinary (baryonic) matter. Referenced in outer-space as accounting for most of the universe's remaining mass-energy alongside dark-energy.

Evidence

Multiple independent observational lines support dark matter's existence:

Galaxy rotation curves — Stars in the outer regions of spiral galaxies orbit faster than expected if only visible matter were present. Flat rotation curves imply large amounts of unseen mass in extended halos. First noted by Babcock (1939) for Andromeda; confirmed across hundreds of spirals by the 1980s–90s.

Gravitational lensing — Massive objects bend light from background sources. The measured distortion of background galaxies by galaxy clusters implies far more mass than visible matter accounts for. Strong lensing (arcs) and weak lensing (minute shear) are both used. Dark matter outweighs visible matter by approximately 5:1 in galaxy clusters.

Galaxy cluster mass estimates — Three independent methods (velocity scatter, X-ray gas temperature, gravitational lensing) consistently agree that clusters contain ~5× more dark matter than baryonic matter.

Cosmic microwave background — Anisotropy patterns in the CMB constrain the total matter density, requiring significant non-baryonic matter.

Theoretical Classifications

Dark matter candidates are classified by their velocity distribution in the early universe:

• Cold dark matter (CDM) — Free streaming length much smaller than a dwarf galaxy protogalaxy. Implies small structures form first, then cluster into larger ones. Consistent with deep-field observations showing galaxies formed before clusters. The dominant accepted model.
• Warm dark matter (WDM) — Intermediate free streaming length.
• Hot dark matter (HDM) — Free streaming length much larger (e.g., neutrinos). Implies large pancake-like structures form first; ruled out by galaxy formation observations.

The Lambda-CDM model incorporating cold dark matter is the standard cosmological model.

Candidate Particles

The leading particle candidate class is WIMPs (weakly interacting massive particles) — stable, electrically neutral, weak-scale particles whose relic abundance after freeze-out in the early universe matches observed dark matter density. No WIMP has been directly detected. Other candidates include axions, sterile neutrinos, and primordial black holes.

Open Problem

The nature of dark matter remains one of the major unsolved problems in physics and astronomy. It has not been directly detected in laboratory experiments despite decades of searches.