Constraints on PBH as dark matter from observations: a review

Primordial black holes (PBHs) are a fascinating candidate for being the dark matter, albeit one which has been heavily constrained. This review presents an in depth look at those observational constraints, particularly at their nuances and uncertainties. Despite their varied origins, the standard PBH formation path is assumed to be collapse of perturbations after inflation, which should leave signals visible in the CMB at certain scales. Other constraints come from microlensing surveys, which severely limit PBHs as dark matter in the solar to satellite range, but there are diminishing results in regards to lower mass ranges. Gravitational waves signals and PBH evaporation from Hawking radiation also make for useful probes, but the former requires the next generation of experiments before making constraints beyond the solar mass range, and the later is severely limited above $10^{-16} \rm M_{\odot}$. Other dynamical and accretion constraints exist for PBH of large masses. Care also has to be given, as all these constraints can carry different implications coming from differences between monochromatic and extended mass distributions, and their degree of clustering. Beyond all these issues, a window still exists for primordial black holes to be all of the dark matter between $10^{-16}$ and $10^{-11} \rm M_{\odot}$