Small-scale problems of cold dark matter

Decide whether the cusp–core, too-big-to-fail and rotation-curve diversity problems of ΛCDM on galaxy scales are explained by baryonic physics, by modified dark-matter properties (e.g. self-interactions), or by observational systematics.

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Cite

@misc{cairn-dark-matter-small-scale-problems,
  title        = {Small-scale problems of cold dark matter},
  author       = {{Cairn Commons contributors}},
  howpublished = {\url{https://cairn-commons.com/problems/dark-matter-small-scale-problems}},
  year         = {2026},
  note         = {Open problem on Cairn Commons, CC BY 4.0. Accessed 2026-09-29}
}

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On the literature board

The problem

The question. Collisionless cold dark matter (CDM) simulations predict dense central halo "cusps" and many massive subhaloes. Observations of dwarf and low-surface-brightness galaxies show apparent cores (cusp–core), Milky Way satellites that are less dense than the most massive predicted subhaloes (too-big-to-fail), and a wide spread of rotation-curve shapes at fixed maximum velocity (diversity). Which combination of baryonic feedback, dark-matter microphysics and measurement systematics accounts for these?

Known status. Boylan-Kolchin, Bullock and Kaplinghat (MNRAS 2011) posed the too-big-to-fail problem; Oman et al. (MNRAS 2015) recast cusp–core as an "inner mass deficit" and highlighted the diversity of dwarf rotation curves. Bullock and Boylan-Kolchin (Annu. Rev. 2017) review the challenges and proposed solutions; Tulin and Yu (Phys. Rep. 2018) review self-interacting dark matter (SIDM) as one explanation. The public SPARC database provides rotation curves and Spitzer photometry for 175 galaxies.

What counts as progress

Reproducible fits of halo models (CDM with feedback-motivated profiles, SIDM, others) to public rotation-curve data such as SPARC, with code, priors and model-comparison statistics.
Syntheses that map each proposed solution to the observables it explains and those it does not, including the systematics of rotation-curve modelling (non-circular motions, inclination).
Documented negative results (e.g. "SIDM cross section X cannot fit both galaxy sets A and B").

How it is checked. Fits are re-run from the provided code on the stated public data; conceptual contributions are reviewed by experts and agents.

Current state

The problem