Too-big-to-fail problem

The mismatch between simulated massive dark-matter subhalos and the lower central densities inferred for observed satellite galaxies. It challenges predictions of structure formation on small scales.

Dark matter halo: A gravitationally bound region of dark matter surrounding a galaxy or larger structure. The problem concerns the predicted inner structure of halos hosting satellite galaxies.
Cold dark matter: A model in which dark matter consists of slowly moving particles that form structure through gravitational clustering. The predicted abundance and structure of subhalos arise in this framework.
Core–cusp problem: The disagreement between constant-density dark matter cores inferred in some galaxies and cuspy centers predicted by cold dark matter. It concerns inner density profiles, while too-big-to-fail concerns which subhalos host observed satellites.
Reionization: The era when ultraviolet radiation ionized most hydrogen in the intergalactic medium. Heating during reionization can prevent low-mass subhalos from forming visible galaxies.
Subhalo: A gravitationally bound dark matter structure orbiting within a larger halo. Simulations predict subhalos whose central densities exceed those inferred for known satellites.
Via Lactea simulation: A high-resolution cosmological simulation of the formation of a Milky Way-sized dark matter halo. Its resolved subhalo population helped expose the density mismatch.
Missing satellites problem: The original discrepancy between the many subhalos predicted by cold dark matter and the smaller number of observed satellite galaxies. It counts luminous satellites; too-big-to-fail focuses on the densities of the largest predicted subhalos.
Stellar feedback: The transfer of energy and momentum from stars into surrounding gas through radiation, winds, and supernovae. It can alter dwarf-galaxy formation and the central densities inferred from their stars.
Circular velocity: The orbital speed required for a circular orbit at a given radius in a gravitational potential. Peak circular velocity is used to identify the simulated subhalos that seem too dense.
Aquarius Project: A suite of high-resolution simulations of Milky Way-mass dark matter halos. Its subhalo predictions sharpened comparisons with the Milky Way's dwarf satellites.

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