Black holes are among the most intriguing and fundamental objects in the universe, serving as crucial probes of both astrophysical phenomena and the fundamental nature of physics. They provide a unique opportunity to test the predictions of general relativity under extreme conditions. Since the advent of gravitational wave astronomy in 2015, the detection of approximately 100 binary black hole mergers has significantly advanced our understanding of these enigmatic objects.

My talk will explore what we have learned about black holes since the dawn of gravitational wave astronomy. Before 2015, theoretical models offered widely varying predictions regarding their rates and properties. I will begin my talk by discussing how observational data have since enhanced our understanding of their population distribution across the universe. Next, a significant portion of my presentation will focus on using gravitational waves as a probe for testing of strong-field gravity near black holes. I will also discuss how gravitational wave observations allow us to examine whether these compact object mergers conform to the theoretical predictions of black holes in general relativity. Specifically, I will also discuss how gravitational wave observations allow us to probe the nature of black holes and the question of whether the objects identified as black holes are truly such—or if they could instead be unknown dark compact objects. These investigations are critical for assessing the validity of general relativity in regions of extreme spacetime curvature and for deepening our understanding of black holes themselves. Finally, I will conclude with an outlook on the future of gravitational wave astronomy, highlighting its potential to further our knowledge of black holes.