The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales of galaxies. Indeed, N-body simulations predict some galaxy properties that are at odds with the observations. These discrepancies are primarily related to the dark-matter distribution in the innermost regions of the halos of galaxies and to the dynamical properties of dwarf galaxies. They may have three different origins: (1) the baryonic physics affecting galaxy formation is still poorly understood and it is thus not properly included in the model; (2) the actual properties of dark matter differs from those of the conventional cold dark matter; (3) the theory of gravity departs from General Relativity. Solving these discrepancies is a rapidly evolving research field. We illustrate some of the solutions proposed within the cold dark-matter model, and solutions when including warm dark matter, self-interacting dark matter, axion-like particles, or fuzzy dark matter. We also illustrate some modifications of the theory of gravity: Modified Newtonian Dynamics (MOND), MOdified Gravity (MOG), and $f\left(R\right)$ gravity.

中文翻译：

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星系规模的暗物质

冷暗物质模型成功地解释了大规模宇宙结构的出现和演化，并且当我们包含宇宙常数时，还解释了均质和各向同性宇宙的性质。但是，冷暗物质模型在银河系尺度上面临持续的挑战。实际上，N体模拟预测了一些与观测结果不一致的星系特性。这些差异主要与星系晕最内层区域的暗物质分布以及矮星系的动力学特性有关。它们可能具有三个不同的起源：（1）影响星系形成的重子物理学仍然知之甚少，因此没有适当地包含在模型中；（2）暗物质的实际性质不同于常规冷暗物质的性质；（3）重力理论与广义相对论背道而驰。解决这些差异是一个快速发展的研究领域。我们举例说明了冷暗物质模型中提出的一些解决方案，以及包括暖暗物质，自相互作用暗物质，轴突样颗粒或模糊暗物质时的解决方案。我们还说明了引力理论的一些修改：修改的牛顿动力学（MOND），修改的重力（MOG）和$F（\mathrm{[R}）$ 重力。