Abstract. With the discovery of gravitational waves from merging pairs of massive black holes, the interest in the question of whether Primordial Black Holes (PBHs) could constitute Dark Matter (DM) has recently been revived. In this talk, I will review the different mechanisms for (DM) PBH formation by focusing on the excursion set theory. I will also explain our recent work on the merger history of DK PBHs in a core of typical dwarf galaxies. In this work, we investigate the possibility of a sequence of collisions and the final mass spectrum in such clusters. Our study shows that the merging process results in the transfer of about 40% of the total mass of the core to the merger products regardless of the initial mass of PBHs, with about 5% of energy radiated out in the form of gravitational waves.