Several decades ago advances in astronomical instruments opened new vistas on the Universe. Black holes popped out, sometimes glowing brightly in the darkness, other times remaining hidden except for ...Several decades ago advances in astronomical instruments opened new vistas on the Universe. Black holes popped out, sometimes glowing brightly in the darkness, other times remaining hidden except for their influence on visible neighbors. In this section we look at how black holes are detected and how they affect the Universe and its contents.