With the discovery of gravitational waves by LIGO-Virgo, black holes have regain interest in the recent years. In particular primordial black holes (PBHs), which originate from the very early Universe, may constitute (at least in part) dark matter. The possibility that dark matter is made of black holes is particularly appealing, and multi-messenger searches are important to probe this hypothesis. During this seminar, I will review the concept of primordial black holes, and discuss their origins and the current contraints. In addition, I will show that the study of black holes is of utmost interest since they may constitute portals to new physics and to quantum gravity.