While we can’t see many things, there is evidence for their existence by the observable effect that they have on other things. The same is true of black holes.
Big black hole and its bright quasar raise questions about the early universe.
Is our universe inside a black hole belonging to another universe?
Black holes are real phenomena in our universe.
An animation of a black hole gravity lens
Black holes may or may not exist. If they do exist, they are invisible, weigh billions of tons, and are smaller than a kernel of corn!