Black holes are formed when stars ( old stars ) explode.
During the explosion, the components which get seperated from the star join again but forms black holes. The bigger the star , the bigger the black hole.
When the black holes are formed ,they are open from one direction and from that sucks all .
Black are one of the greatest mysteries of the cosmos.

Till then I think my answer satisfies u
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but whats the link between dark matter n dark energy?
actually Suddha , dark energy is emitted by dark matter itself. There is matter of which earth and all of us are made. Matter can release energy but dark matter is the opposite to matter.
okay... i got it. thank you
no problem