Dark Matter Candidates and Particle Models

Many theories propose what dark matter might be, from weakly interacting massive particles to ultralight axions, each implying a different way it could be discovered.

Definition

Dark matter candidates are hypothesized particles or objects that could constitute the non-baryonic dark matter, characterized by their mass, interaction strength, and the cosmological mechanism that produces the observed relic abundance.

Scope

This topic covers the leading particle candidates for dark matter, including weakly interacting massive particles, axions, sterile neutrinos, and primordial black holes, the production mechanisms that set their relic abundance such as thermal freeze-out, and the theoretical frameworks like supersymmetry that motivate them.

Core questions

• What kinds of particles could make up dark matter?
• How does each candidate acquire the right cosmic abundance?
• Why are WIMPs and axions considered especially well motivated?

Key concepts

• WIMP
• Axion
• Sterile neutrino
• Primordial black hole
• Thermal freeze-out
• Relic abundance
• Supersymmetry

Key theories

Thermal relic WIMPs

A weakly interacting massive particle in thermal equilibrium in the early universe freezes out with a relic abundance naturally close to the observed dark-matter density, a coincidence known as the WIMP miracle.

Axions and light candidates

Ultralight bosons such as the axion, originally proposed to solve a problem in particle physics, can be produced non-thermally in the early universe and behave as cold dark matter, offering an alternative to WIMPs.

Mechanisms

Thermal candidates remain in equilibrium until their annihilation rate falls below the expansion rate and they freeze out, fixing the relic density by their cross section; non-thermal candidates such as axions are produced by field oscillations, while the cold, slow nature of viable candidates is required to match structure formation.

Clinical relevance

The candidate landscape guides experimental strategy: the assumed mass and interaction strength determine whether dark matter is best sought in underground detectors, with telescopes hunting annihilation products, in axion haunting cavities, or at colliders, so theory and search programs are tightly linked.

History

After the dynamical evidence matured, the 1980s saw cold dark matter become the standard paradigm, with supersymmetric WIMPs and the axion emerging as leading candidates; decades of null searches have since broadened attention to sterile neutrinos, primordial black holes, and lighter or more weakly coupled particles.

Debates

The status of the WIMP paradigm

The absence of WIMP detections despite increasingly sensitive experiments has prompted debate over whether the WIMP remains the most likely candidate or whether attention should shift to axions, lighter particles, or entirely different scenarios.

Key figures

• Gianfranco Bertone
• Dan Hooper
• Joseph Silk
• Helen Quinn
• Roberto Peccei

Seminal works

• bertone2005

Frequently asked questions

What is the WIMP miracle?

It is the observation that a stable particle with roughly weak-scale mass and interactions, freezing out of thermal equilibrium in the early universe, would naturally end up with about the observed dark-matter density, making WIMPs a theoretically attractive candidate.

Could dark matter be black holes?

Primordial black holes formed in the early universe are a possible candidate, but observational constraints from lensing, dynamics, and the cosmic microwave background limit them to narrow mass windows, so they can be at most a fraction of the dark matter in most ranges.