Think of a mass so densely compact as not to allow light to escape from it. Such a compact body is called a "Black Hole", and it is indeed very difficult to visualise it. These cosmic beasts are formed when stars above a certain mass die after running out of all their nuclear fuel. Black holes are extremely compact space objects that were once massive stars which collapsed inward due to the force of their own gravity. They pack vast mass into a pinpoint of space.
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