Black Holes are fascinating because they show that there are regions in the Universe from which we cannot extract any information. Because we usually extract information only if we receive something from an object. But, once an object enters a black hole, all information about it seems to be lost. That unsettles a lot of physicists.
In fact, for several decades after people began researching them, black holes were considered as mathematical oddities of the theory, with no possibility of actually existing. All that has changed in the past few decades, and now several tens of black holes have been catalogued!
If we can't see a black hole, as light cannot leave it, how do we detect them!? You will learn about that and much more in the next module.