Ultra-light dark matter is a class of dark matter models (DM), where DM is composed by bosons with masses ranging from \(10^{-24}\, \mathrm {eV}< m < \mathrm {eV}\). These models have been receiving a lot of attention in the past few years given their interesting property of forming a Bose–Einstein condensate (BEC) or a superfluid on galactic scales. BEC and superfluidity are some of the most striking quantum mechanical phenomena that manifest on macroscopic scales, and upon condensation, the particles behave as a single coherent state, described by the wavefunction of the condensate. The idea is that condensation takes place inside galaxies while outside, on large scales, it recovers the successes of \(\varLambda \)CDM. This wave nature of DM on galactic scales that arise upon condensation can address some of the curiosities of the behaviour of DM on small-scales. There are many models in the literature that describe a DM component that condenses in galaxies. In this review, we are going to describe those models, and classify them into three classes, according to the different non-linear evolution and structures they form in galaxies: the fuzzy dark matter (FDM), the self-interacting fuzzy dark matter (SIFDM), and the DM superfluid. Each of these classes comprises many models, each presenting a similar phenomenology in galaxies. They also include some microscopic models like the axions and axion-like particles. To understand and describe this phenomenology in galaxies, we are going to review the phenomena of BEC and superfluidity that arise in condensed matter physics, and apply this knowledge to DM. We describe how ULDM can potentially reconcile the cold DM picture with the small-scale behaviour. These models present a rich phenomenology that is manifest in different astrophysical consequences. We review here the astrophysical and cosmological tests used to constrain those models, together with new and future observations that promise to test these models in different regimes. For the case of the FDM class, the mass where this model has an interesting phenomenology on small-scales \( \sim 10^{-22}\, \mathrm {eV}\), is strongly challenged by current observations. The parameter space for the other two classes remains weakly constrained. We finalize by showing some predictions that are a consequence of the wave nature of this component, like the creation of vortices and interference patterns, that could represent a smoking gun in the search of these rich and interesting alternative class of DM models.

超轻暗物质是一类暗物质模型（DM），其中DM由质量范围为\(10^{-24}\, \mathrm {eV}< m < \mathrm {eV}\的玻色子组成）。这些模型在过去几年中受到了广泛关注，因为它们具有形成玻色-爱因斯坦凝聚体（BEC）或银河尺度超流体的有趣特性。 BEC 和超流性是在宏观尺度上表现出来的一些最引人注目的量子力学现象，并且在凝结时，粒子表现为单一相干态，由凝结物的波函数描述。这个想法是，凝结发生在星系内部，而在外部，在大范围内，它恢复了\(\varLambda \) CDM 的成功。 DM 在星系尺度上凝结时产生的波动性质可以解决小尺度上 DM 行为的一些好奇心。文献中有许多模型描述了星系中凝聚的 DM 成分。在这篇综述中，我们将描述这些模型，并根据它们在星系中形成的不同非线性演化和结构将它们分为三类：模糊暗物质（FDM）、自相互作用模糊暗物质（ SIFDM）和 DM 超流体。这些类别中的每一类都包含许多模型，每个模型都呈现出类似的星系现象。它们还包括一些微观模型，例如轴子和类轴子粒子。为了理解和描述星系中的这种现象，我们将回顾凝聚态物理中出现的 BEC 和超流现象，并将这些知识应用于 DM。我们描述了 ULDM 如何潜在地协调冷 DM 图像与小规模行为。这些模型呈现了丰富的现象学，体现在不同的天体物理后果中。我们在这里回顾用于约束这些模型的天体物理学和宇宙学测试，以及有望在不同制度下测试这些模型的新的和未来的观测。对于 FDM 类的情况，该模型在小尺度上具有有趣现象学的质量\(\sim 10^{-22}\, \mathrm {eV}\)受到当前观察的强烈挑战。其他两类的参数空间仍然受到弱约束。最后，我们展示了一些由于该组件的波性质而产生的预测，例如涡流和干涉图案的产生，这些预测可以代表寻找这些丰富而有趣的替代类 DM 模型的确凿证据。