Primordial Black Holes as Dark Matter: Recent Developments
Although the dark matter is usually assumed to be some form of elementary particle, primordial black holes (PBHs) could also provide some of it. However, various constraints restrict the possible mass windows to $10^{16}$ - $10^{17}\,$g, $10^{20}$ - $10^{24}\,$g and $10$ - $10^{3}\,M_{\odot}$. The last possibility is contentious but of special interest in view of the recent detection of black-hole mergers by LIGO/Virgo. PBHs might have important consequences and resolve various cosmological conundra even if they have only a small fraction of the dark-matter density. In particular, those larger than $10^{3}\,M_{\odot}$ could generate cosmological structures through the seed or Poisson effect, thereby alleviating some problems associated with the standard cold dark-matter scenario, and sufficiently large PBHs might provide seeds for the supermassive black holes in galactic nuclei. More exotically, the Planck-mass relics of PBH evaporations or stupendously large black holes bigger than $10^{12}\,M_{\odot}$ could provide an interesting dark component.