Clusters of primordial black holes
  The Primordial Black Holes (PBHs) are gradually involved into consideration
as the phenomenon having reliable basis. We discuss here the possibility of
their agglomeration into clusters that may have several prominent observable
features. The clusters can form due to closed domain walls appearance in the
natural and the hybrid inflation with subsequent evolution and gravitational
collapse. Early dustlike stages of dominance of heavy metastable dissipative
particles, at which star-like objects are formed, can also naturally lead to
formation of black hole clusters, remaining in the Universe after decay of
particles, from which they have originated. The dynamical evolution of such
clusters discussed here is of the crucial importance. Such a model inherits all
the advantages of the single PBHs like possible explanation of existence of
supermassive black holes (origin of the early quasars), binary BH merges
registered by LIGO/Virgo through gravitational waves, contribution to
reionization of the Universe, but also has additional benefits. The cluster
could alleviate or completely avoid existing constraints on the single PBH
abundance making PBHs a real dark matter candidate. The most of existing
constraints on (single) PBH density should be re-considered as applied to the
clusters. Also unidentified cosmic gamma-ray point-like sources could be
(partially) accounted for by them. One can conclude, that it seems really to be
much more viable model with respect to the single PBHs.
