Baryonic solutions and challenges for cosmological models of dwarf galaxies

Galaxies and their dark-matter haloes have posed several challenges to the dark energy plus cold dark matter (ΛCDM) cosmological model. These discrepancies between observations and theory intensify for the lowest-mass (‘dwarf’) galaxies. ΛCDM predictions for the number, spatial distribution and internal structure of low-mass dark-matter haloes have historically been at odds with observed dwarf galaxies, but this is partially expected, because many predictions modelled only the dark-matter component. Any robust ΛCDM prediction must include, hand in hand, a model for galaxy formation to understand how baryonic matter populates and affects dark-matter haloes. In this Review, we consider the most notable challenges to ΛCDM regarding dwarf galaxies, and we discuss how recent cosmological numerical simulations have pinpointed baryonic solutions to these challenges. We identify remaining tensions, including the diversity of the inner dark-matter content, planes of satellites, stellar morphologies and star-formation quenching. Their resolution, or validation as actual problems with ΛCDM, will probably require both refining of galaxy-formation models and improving numerical accuracy in simulations. Galaxies and their dark-matter haloes have posed several challenges to the dark energy plus cold dark matter (ΛCDM) cosmological model. This Review discusses the most notable challenges to ΛCDM regarding dwarf galaxies and the insights provided by recent cosmological numerical simulations.