Hi , how can we make sure that there is a black hole in universe?

Question

there is no sure sight of black holes , so .........

Answer 1

You are right in that you can not see a black hole. That is because the gravitational field of the object in question is too strong for light to escape. So the object (whatever it may be) is black because it emits or reflects no light. As to whether or not it is a hole is debatable.
There are phenomena in the universe that hint at black holes. Gravity lenses, x-ray sources, super massive invisible companion objects, etc all are things that have huge amounts of mass and should be visible, yet are not. They could be neutron stars or black dwarfs (white dwarfs that have gone cold). But nuclear forces as we know them tell us that there is an upper limit to the amount of mass that a white or black dwarf or even a neutron star can have before the electromagnetic or strong nuclear forces become overwhelmed by gravity. What happens then is (as you can imagine) pretty much unknown. All we do know is that the resulting object's gravity field creates a situation where the escape velocity exceeds the speed of light, and therefore the object appers black.
To date, the best we can do to show the existence of black holes is show how the predictions that black holes bring to light (pardon the pun) are all satisfied by our present understanding of science.