People can't see black holes as there is no light. It is visible only through the space telescope. It can be seen that stars that are close to black holes act differently than other stars.
There is something within a black hole called a singularity. A singularity is something where all the matter gets squeezed into the black hole. The black hole has a boundary called the event horizon. Here the escape speed is more than the speed of light. The gravity is just strong enough to prevent light from escaping.
Hence, it is impossible to escape its gravity. A black hole is known to be empty. Even if we send someone to explore Black Hole, it might be possible that we won't be able to communicate with the person.
There is an accretion disk which is just outside the black hole. When a black hole consumes a star, the angular momentum of some gases is high enough that they don't go directly inside the black hole but instead spiral around it, looking like a glowing disk around a black hole.
It is wrong to say anything particular what is inside a black hole as nobody has visited it. But according to the theory, it is said that Blackhole has a point of infinite density at its center.
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