Black hole is formed when a massive star collapse at the end of its life cycle. The gravity is so strong that particles and electromagnetic waves such as light cannot escape from inside. It reflects no light. The boundary where no escape is possible is called the event horizon. Matter that falls onto a black hole can been seen to be very bright as it forms an external accretion disk due to heat by friction

It will grow absorbing masses nearby. Generally speaking supermassive black holes are in the centers of most galaxies. (Credit:space-facts.com)