If a part of the dark matter were made of black holes, gravitational lensing offers a way to detect them. A source of light behind any black hole will be gravitationally deflected, resulting in a magnification of the apparent brightness of the source that occurs as a brightening event lasting for a time that may vary from minutes to months. For many years, this method has been used to search for black holes in the halo of our Milky Way galaxy, and strong limits have been set after searching for stars showing such magnification events.
Other types of magnified stars have been seen in clusters of galaxies, where the large cluster mass deflects the light of a background galaxy and distorts their images into highly elongated shapes. In these elongated images, highly magnified individual stars in the galaxy have been detected recently with the Hubble Space Telescope and the James Webb Space Telescope, as were predicted back in 1991. Using several highly magnified stars found so far in several clusters, we have shown with student Claudi Vall Müller how the presence of black holes in the dark matter of these clusters would widen the distribution of the magnified images around the critical lines of the cluster gravitational lens, where the magnification reaches maximum values. Using the distribution of only 9 observed highly magnified images of background stars in various lensing clusters, we put a limit to the fraction of the dark matter that could be in the form of black holes of 2%, which is already comparable to the best limits that have been obtained for the dark matter in the Milky Way Galaxy from the traditional lensing event methods. Upcoming observations of the James Webb Space Telescope are promising to offer a powerful probe to discover an even smaller fraction of black holes or the presence of other smallscale irregularities in the distribution of dark matter in lensing clusters of galaxies.
