What does a black hole look like? Is it invisible of spherical?
It looks like a big black dot.
The existence of black holes goes back many decades however it wasn't until recently astronomers figured out how to see them. They quite simply look for a black hole to pass in front of something emitting light.
A black hole is a gravitational disturbance in the fabric of the universe. It is thought they are made by the largest of stars collapsing beyond the point where they turn into a literal lump of rock and minerals. The reason being that the amount of gravity created is so extreme that nothing, not even light, can escape from it.
But the true nature of black holes remains uncertain, mostly because scientists are very limited in the ways that they can be studied.