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.

星系及其暗物质晕对暗能量加冷暗物质（Λ CDM）宇宙学模型提出了若干挑战。对于最低质量（“矮”）星系，观测和理论之间的这些差异加剧了。Λ CDM 对小质量暗物质晕的数量、空间分布和内部结构的预测历来与观测到的矮星系不一致，但这是部分预期的，因为许多预测只模拟了暗物质成分。任何稳健的Λ CDM 预测都必须包括一个星系形成模型，以了解重子物质如何填充和影响暗物质晕。在这篇评论中，我们认为Λ面临的最显着挑战关于矮星系的 CDM，我们讨论了最近的宇宙学数值模拟如何确定重子解决方案来应对这些挑战。我们确定了剩余的紧张局势，包括内部暗物质含量的多样性、卫星平面、恒星形态和恒星形成淬火。他们的解决方案，或作为Λ CDM实际问题的验证，可能需要改进星系形成模型和提高模拟中的数值准确性。