Primordial black holes are too scarce to explain dark matter

Primordial black holes (PBHs) would have to be at least ten times more abundant to support the theory that they make up a substantial proportion of the dark matter in our Galaxy. Researchers looked for black holes that formed during the early history of the Universe by observing stars in the Large Magellanic Cloud (LMC). A 20-year survey called the Optical Gravitational Lensing Experiment (OGLE) found 13 candidate signals. The detections involved an effect known as microlensing, in which a PBH (or other object) deflects and distorts the light from a star located farther away than the PBH, producing multiple magnified images of the star. The copies cannot be distinguished because they are too close to each other, but their combined brightness shows a characteristic increase as the PBH passes through the line of sight of the star (upper panel).