Observational constraints for primordial mini black holes

In the last ten or so years, the existence of stellar mass black holes and supermassive black holes has been proved beyond any reasonable doubt. We are confident that we understand astrophysical processes that form these two kinds of black holes. It was suggested that the primordial black holes (PBH), with much smaller masses, were formed in the very early universe just after the Big Bang, and that they may contribute to the dark matter halo of our Galaxy. The only direct observational constraints for these hypothetical objects (DMPBH) follow from no detection of any clear signature of the predicted Hawking’s evaporation of them at masses ~1015 g, and from interpretation of the microlensing results at masses >1026 g. Thus, the mass region 1015 g < M < 1025 g remains observationally unconstrained. The question whether (the hypothetical) collisions of DMPBH in this mass range with Earth, white dwarfs, neutron stars and red giant stars could lead to potentially detectable observational signatures, has not yet received a definite answer. In my opinion, such detections are unlikely.