By Doug Davey
The first step in understanding what a black hole is to understand that mass is mostly empty space. Mass is made of atoms, which is mostly empty space. If you can picture all the mass of an enormous star falling so that there is very little empty space, that is a black hole. This mass is so massive that it creates the largest, most violent, gravitational pull in the know universe.
This question is much more complicated then it seems. The gravitational pull is so strong in black holes, that nothing, not even light, can escape. Since nothing escapes, there is no way to detect anything visible, or invisible, from black holes. We can only "see" black holes by seeing how they affect other objects.
Yet another complicated question. This depends on what you are measuring. The area that all of the mass is occupying very little space, one such black hole is 3 billion times more massive then our sun, but takes up less area then our solar system. That is, even if it didn't attract all of the light, it is still too small to see. However, if you are measuring the amount of area that is completely trapped by the black hole, then you are looking for the event horizon. The event horizon is the sphere that is the point that light can't escape. The event horizon varies, depending on how much mass is trapped inside of the black hole. The more mass that is present, the more gravity there is, thus the larger volume the event horizon takes up. Yet another measure of how big a black hole is is the mass. One black hole resides (probably) in the M87 galaxy. The mass of this black hole is calculated to be 3 billion times the mass of our sun.
It is commonly believed that there is a black hole at the center of our galaxy, the Milky Way. Sounds scary, doesn't it. Not to worry, it is too far away to produce any noticeable effects that any person on earth could notice. It is also believed that there are black holes at the centers of a lot of galaxies. There may be some black holes that don't have a galaxy around them.
That last paragraph sounds very vague, with good reason. Since it is impossible to observe a black hole directly, and difficult to observe them indirectly, the best way to observe them is with mathematics. This way it can be calculated where there should be a black hole, judging by the gravitational effects on others.