First I should tell you what a black hole is. The existence of black holes seems to be an unavoidable prediction of Einstein's theory of gravity. A black hole is a region of space so tightly packed with matter, that nothing, not even light can escape. The inside of the black hole is separated from the rest of the Universe by a perfectly round, spherical surface called the event horizon of the black hole. This surface is literally a horizon: anything that goes past the event horizon into the hole disappears from view forever, since, according to Einstein, no signal can escape from inside the horizon to the outside world. It is a one way boundary: things can go in, but nothing can come out. From the outside, a black hole therefore is simply a very round, very black region of space. In some ways it is the most perfect object in the universe, with no features detectable from the outside to interfere with its symmetry.

If this were the whole story, black holes would be quite boring, and extremely difficult to detect. Luckily for observational astronomers, black holes are not expected to exist in total isolation. They are formed by the collapse of ordinary matter, such as dust, stars, or entire galaxies. Real black holes will therefore be surrounded by gas, and other stars, which makes them easier to detect.