Dark Matter Theories to Know

Why This Matters

Dark matter makes up roughly 27% of the universe's total mass-energy content, yet we've never directly detected it. You're being tested on your understanding of why we think dark matter exists (gravitational evidence like galaxy rotation curves and gravitational lensing) and how different theoretical frameworks attempt to explain it. The theories you'll encounter here represent fundamentally different approaches: particle physics candidates, modifications to gravity itself, and exotic cosmological objects.

Don't just memorize names and definitions—know what problem each theory solves and where it struggles. Exam questions often ask you to compare approaches or evaluate which theory best explains a specific observation. Understanding the underlying physics—whether a theory invokes new particles, altered gravitational laws, or primordial structures—will help you tackle both multiple choice and FRQ prompts with confidence.

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Particle Dark Matter Candidates

Most mainstream dark matter research focuses on undiscovered particles that interact gravitationally but barely (if at all) through other forces. The key distinction among particle candidates is their mass and velocity, which determines how they clump and influence structure formation.

Cold Dark Matter (CDM)

• Slow-moving, massive particles that clump efficiently under gravity—the foundation of the standard cosmological model ($\Lambda$CDM)
• Dark matter halos form around galaxies, extending far beyond visible matter and explaining flat rotation curves where $v(r) \approx \text{constant}$
• Large-scale structure predictions match observations of galaxy clusters and cosmic web filaments, making CDM the benchmark theory

Weakly Interacting Massive Particles (WIMPs)

• Hypothetical particles with mass $\sim 10–1000 \text{ GeV}$ that interact only via gravity and the weak nuclear force
• Thermal relic abundance—WIMPs "freeze out" in the early universe at densities matching observed dark matter, called the WIMP miracle
• Direct detection experiments (LUX, XENON, LZ) search for rare WIMP-nucleus collisions, though no confirmed detection yet

Axions

• Ultra-light particles ($m \sim 10^{-5} \text{ eV}$) originally proposed to solve the strong CP problem in quantum chromodynamics
• Produced non-thermally in the early universe, potentially forming a cold, coherent field rather than individual particles
• Detection via photon conversion—axions can convert to photons in strong magnetic fields, enabling searches like ADMX

Sterile Neutrinos

• Right-handed neutrinos that don't interact via standard weak force, with masses in the $\text{keV}$ range making them warm dark matter candidates
• Produced through oscillation with active neutrinos in the early universe—abundance depends on mixing angle
• X-ray signatures could reveal their decay; the unexplained 3.5 keV line in galaxy clusters sparked interest (though remains controversial)

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Alternative Particle Frameworks

Some theories modify how dark matter particles behave or introduce entirely new physics beyond the simplest cold, collisionless model.

Self-Interacting Dark Matter (SIDM)

• Dark matter particles scatter off each other with cross-section $\sigma/m \sim 1 \text{ cm}^2/\text{g}$, unlike collisionless CDM
• Solves the core-cusp problem—interactions redistribute energy, creating observed flat density cores instead of CDM's predicted steep cusps
• Preserves large-scale success of CDM while improving small-scale predictions for dwarf galaxies and galaxy clusters

Warm Dark Matter

• Intermediate velocity particles with masses $\sim \text{keV}$—faster than CDM, slower than relativistic "hot" dark matter
• Free-streaming erases small structures—particles travel far before clumping, suppressing formation of the smallest halos
• Reduces satellite galaxy overabundance—CDM predicts more dwarf galaxies than observed; warm dark matter naturally produces fewer

Fuzzy Dark Matter

• Ultra-light bosons ($m \sim 10^{-22} \text{ eV}$) with de Broglie wavelengths on galactic scales, exhibiting quantum wave behavior
• Suppresses small-scale structure through quantum pressure, addressing the same problems as warm dark matter via different physics
• Solitonic cores form at halo centers—stable, wave-interference patterns that could explain observed flat density profiles

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Non-Particle Approaches

Not all dark matter theories invoke new particles. Some modify gravity itself or propose that dark matter consists of macroscopic objects.

Modified Newtonian Dynamics (MOND)

• Alters Newton's second law at very low accelerations ($a < a_0 \approx 1.2 \times 10^{-10} \text{ m/s}^2$), eliminating the need for dark matter in galaxies
• Successfully predicts rotation curves using only visible matter—the Tully-Fisher relation emerges naturally from MOND
• Struggles with galaxy clusters and cosmology—cannot fully explain gravitational lensing or CMB observations without some dark matter

Primordial Black Holes

• Black holes formed in the early universe from density fluctuations, not stellar collapse—masses could range from $10^{-18} M_\odot$ to thousands of $M_\odot$
• Gravitational detection through microlensing surveys and gravitational wave observations from mergers (LIGO/Virgo)
• Constrained but not ruled out—certain mass windows remain viable, particularly asteroid-mass ($10^{17}–10^{23} \text{ g}$) primordial black holes

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Exotic and Speculative Models

These theories push beyond mainstream approaches, proposing entirely new sectors of physics.

Mirror Dark Matter

• Parallel "mirror sector" of particles duplicating the Standard Model—mirror protons, electrons, photons interacting among themselves
• Explains dark matter's invisibility—mirror particles interact gravitationally with ordinary matter but have their own separate forces
• Could address matter-antimatter asymmetry if mirror sector evolved differently, providing insights into baryogenesis

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Quick Reference Table

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Self-Check Questions

1. Which two particle candidates are both produced in the early universe but differ dramatically in mass—one at $\text{GeV}$ scales, one at $10^{-5} \text{ eV}$? What detection methods apply to each?

2. Compare and contrast how Warm Dark Matter and Fuzzy Dark Matter each address the small-scale structure problems of standard CDM. What's the key physical mechanism in each case?

3. A galaxy's rotation curve is flat at large radii. Which theories could explain this observation, and which would invoke new particles versus modified physics?

4. Why does MOND successfully predict individual galaxy rotation curves but struggle with galaxy cluster observations? What does this suggest about the theory's limitations?

5. If primordial black holes in the asteroid-mass range ($10^{17}–10^{23} \text{ g}$) constitute dark matter, how might we detect them, and why have other mass ranges been constrained?