Primordial Black Holes as Dark Matter

What is the nature of dark matter? This unknown part of the Universe's content is to the best of our current understanding about five times as abundant as ordinary matter. Despite decades of intense research, its characteristics still remain mysterious. Yet, it is essential for explaining the structure in our Universe. The usual assumption is that the dark matter comprises weakly-interacting particles. No evidence for these has been found so far.

I will discuss another well-motivated dark matter candidate: black holes produced in the early Universe, so-called primordial black holes. These could have seeded any of the observed black holes, such as the supermassive black holes in galactic centres and those detected through gravitational waves (2017 & 2020 Nobel Prizes in Physics, respectively). Furthermore, these macroscopic objects are natural dark matter candidates as — a priori — they would not require new particles or interactions; the very same mechanism which generates cosmic structure may also generate black holes. To date, there are already numerous hints for the existence of these fascinating objects, which I will touch upon in this lecture.