I attended a lecture by Dr. Charles D. Bailyn of Yale University on how to find a black hole. I learned a few things I didn't know before. First, Black Holes are only theoretical. You assume they exist only if you assume Einstein’s General Theory of Relativity to be true. Second, I had always heard the term "Event Horizon" but never really knew what it meant. In one of these Theoretical Black Holes the escape velocity is greater than the speed of light, thus the term Black Hole. So matter, energy, and even light is sucked down into the core of a Black Hole. Since light cannot escape, there is no way to "observe" one. But the further you move away from the center of the Black Hole (Singularity - A point of infinite density) the less the escape velocity has to be. Anyway, Event Horizon is when the escape velocity equals the speed of light. This creates sort of an end point, a boundary to the Universe if you will. I ring of light surrounding a Black Hole. Black Holes are in effect a place where nothing can escape. So, will the universe collapse in all of these Black Holes? Nope, the Universe is expanding and the Black holes are rushing away from one another. How do you observe these? The technique Dr. Bailyn uses is to look at a black hole in close proximity to another star, a two star system. Here the black hole will tear off gasses from the partner star and then emit X-ray flashes. We can observe these X-ray flashes and then determine where a Black Hole might be. We can then measure the rotational velocity of the gasses or the Star around the Black Hole. If it is a Black Hole, this velocity will be huge in comparison to other two star systems. If these criteria are met, then you may have a Black Hole. Currently we think there are around 20 Black Holes in our Galaxy, the Milky Way. How are they created? It is the natural end of life of every star.
Dr. Bailyn Website http://www.iau.org/administration/membership/individual/10503/