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Hi there,
black hole is a very very dense singularity ie,1000 earths packed into a very small space.due to this the gravitational force exerted by this singularity is very strong that light cannot pass through it.
they are formed when a massive  star dyes ,or due to a massive supernova explosion.these stars eventually end their lives by an explosion.

event horizon in astronomy means a point  where there is no return.
in black hole ,when an  object enters a black hole a certain point is reached when there can be no return .that is why we cannot see a black hole,the light traveling near it gets sucked up ,so it stays invisible until some object gets absorbed.
i hope this helps u...

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We can say that a black hole is a continuously eating machine which can't be stopped. till now only a black hole is a single thing that doesn't reflect light. it doesn't reflect light because if a ray of light gets in then it doesn't return. this phenomenon of black hole is due to the massive gravitational pull. a black hole is formed when a star blasts. it's gravitational pull is this much that its like gravitational pull of 1000 earth in a thing equal to our nail's size. its gravitational pull is so powerful that it breaks the light particles that's why even light cant pass throuh it. the black holes can't be seen because they don't reflect light. their presense an only be detected by the effect on things around it.
a act hole works like this....
it's like a rotating disc. the body present near it starts to break down into particles and get into it. when a thing enters it black hole's gravitational pull crushes the object and particles are thrown out from centre in upward direction as well as in downward direction.
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