When a star runs out of fuel it collapses in to it's self. Sins the star is so hot when it does this it explodes!
After doing this most of the star gets shot in to space but a small amount of the core stays.
When this happens the small amount of core has a great deal of mass. (Which is the gravitational pull.) The mass pulls every thing around it in to it and it gets so very dark because no light can escape the gravity.
And so it's called the Black Hole.
Scientists see how Black Holes effected the surrounding things. For example gasses move very quickly when there is a Black Hole near.
The universe is getting bigger and bigger and so we are moving away from any Black Holes.