… so search called off. In 1971, Stephen Hawking suggested that dark matter consists of primordial black holes formed when dense regions of the early universe underwent gravitational collapse. Such holes were not observed, and one reason could have been small size (less than our Moon). However, from Nature News:
In a paper posted to the arXiv preprint server on 13 January3, astrophysicists Paolo Pani of the University of Lisbon in Portugal and Avi Loeb of the Harvard?Smithsonian Center for Astrophysics in Cambridge, Massachusetts, considered what would happen if small primordial black holes happened to pass through neutron stars, which can be observed from Earth. The researchers suggest that despite their small mass, the black holes would destroy the stars. That means that the presence of neutron stars can be used to place limits on the prevalence of small black holes that would have destroyed them if they existed. “We are using neutron stars as detectors for primordial black holes,” says Loeb.
Pani and Loeb find that the prevalence of neutron stars in the Milky Way almost entirely closes the mass range left open by the 2013 study, meaning primordial black holes, even tiny ones, are unlikely to exist in numbers sufficient to explain dark matter.
Most researchers have assumed that dark matter is actual particles, not black holes, and the search for them continues.