A black hole is a section of space that has a gravitational force solid enough to prevent anything, as well as light, from escaping. The escape velocity of a star or planet is defined as, how fast an object must be moving to escape its gravitational field. Since the singularity of a black hole has infinite density, the attraction of its gravity is so tough that the escape velocity is more than even the speed of light. For instance the speed of light is as well the speed of all of the electromagnetic spectrum, no rays (counting gamma, X-rays, visible light and radio waves) can outflow. This is the reason why black holes are invisible to us.
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