Today we know much about the sky: how stars are born, how they live and die, and how the universe as a whole evolves. We have learned of the existence of another type of matter, indifferent to light and yet decisive for the formation of galaxies, and we have a hint of a dark energy that since the last 4.5 billion years has taken over the control of the cosmos. We postulated and then discovered and even photographed black holes and listened to the...