Black holes are part of the universe and some beliefs circulate around them. The most common is that they are huge holes in the middle of the cosmos that 'swallow' everything in their path, taking it to a deep infinity that no one knows.
The explanation issued by NASA is that black holes are not really holes in themselves and they are not empty. They are cold remains of very old stars that contain a great amount of matter in a compact space. This is due to their powerful gravitational force. The density of black holes is so great that no material particle, not even light, is able to free itself from the surrounding gravitational force.