2015 Khlopov 2019), up to astrophysical size primordial black holes (for a review on recent bounds, see (Carr et al.
It spans more than 80 orders of magnitude and shows very different hypothesis for DM, from new elementary particles, to composite objects (Jacobs et al. This figure shows many different broad classes of DM models, and each of which might contain many different specific models. This incredible variety of viable models of DM can be seen in the huge range of masses those models cover, as shown in Fig. Those models recover the large scale properties of CDM, but invoke very different objects and phenomena to play the role of DM. This allows for the creation of a plethora of possible models of DM. However, even though we know the hydrodynamical properties of DM on large-scales to a very high precision, the microphysics of the DM component remains unknown. This coarse-grained description of a CDM is very successful in fitting the linear, large-scales observations from the CMB, LSS, to clusters, and general properties of galaxies. In the CDM model, DM is described by a perfect fluid that must be massive, sufficiently cold, which means non-relativistic at the time of structure formation, and collisionless to explain the observational data on large linear scales. Therefore, within \(\varLambda \)CDM, the Cold Dark Matter (CDM) paradigm emerged from the large-scale observations and describes the component responsible for the formation of the structures of our universe through gravitational clustering. Dark energy is parametrized by a cosmological constant, the simplest model for the present accelerated expansion of our universe. Ultra-light dark matter is a class of dark matter models (DM), where DM is composed by bosons with masses ranging from \(10^ \approx 0\), that does not interact, at least strongly, with baryonic matter.
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