Black holes are regions of space that have a strong gravitational pull that even light can escape. It is therefore impossible to spot them and, moreover, information on what happens within its gravitational field. This is because the amount of matter they have is immense. For example, a black hole the size of the sun is millions of times more dense than the star. Some of the black holes are called primordial, emerged with the formation of the universe. Others are the product of the extinction of giant stars, called supernovas.
The black holes can not be seen because they do not emit or reflect light. But astronomers know of their existence by the behavior of matter found in its vicinity, in general, are attracted by them.
No. But the Hubble Space Telescope showed a spot bright in the heart of the elliptical galaxy M87. It is believed that this spot is formed by gases, without absorbed by a black hole.