Evidence has recently piled up that there is a gigantic black hole, Sagittarius A*, with a mass 4 million times that of our Sun, at the center of our galaxy. Similar black holes appear to exist at the center of most galaxies. Some are even much bigger—with masses billions times that of our Sun. Can you imagine a black hole a billion times the size of our Sun?
The existence of these giants, once again, changes our picture of the universe. Clearly these monsters must have played a major role in the history of the cosmos, but we do not know how. Astronomers are building a "telescope" as large as the Earth, connecting in line many existing radio telescopes to actually see Sagittarius A* directly. It is called the "event horizon" telescope.
But these immense holes are also the boundary of our current knowledge: we see matter falling into them—we have no idea what ultimately happens to it. Space and time appear to come to an end inside. Or, better said, to morph into something we do not yet know. The universe is still full of mystery.