What is a black hole and have any been found?

The more dense a star or planet is, the harder it is to escape from its 'gravitational pull'. If an object is sufficiently dense, then nothing, not even light, can escape its gravitational field. In this case, the object is a black hole - black because no radiation can escape. To get some idea of the incredible densities required, the whole mass of the earth would have to be compressed into a sphere with a radius of less than one centimetre before it would be dense enough to form a black hole. Nevertheless, it is believed that the required densities can occur at the end of the life cycles of certain stars, and recently, strong evidence has emerged suggesting that supermassive black holes lie at the centre of many galaxies.
By mahesh saagr khanal for scientific nepal

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