Black hole

Black holes are rips in space-time where gravity is so strong that even light cannot escape them, making them appear to be large spheres. In reality, the surface of the sphere is just the point at which light cannot escape, so it appears to be black. This surface of the black hole is called the event horizon.

The center of a black hole is an infinitely small and infinitely dense point within spacetime, usually called a singularity. Increasing the mass of the singularity effects the curvature of spacetime and causes the black hole's event horizon to grow in size.

A non-moving black hole can be described by only it's mass, electric charge, and angular momentum. In other words, two black holes with the same mass, electric charge, and angular momentum would be physically indistinguishable from each other besides their position.

Formation
Black holes are formed when a star burns out and has enough gravity, in which it will then collapse in onto itself, usually creating a supernova. The density inside the center of the star while it collapses becomes enough that a singularity forms, and so a black hole is created. Black holes have also been created naturally by advanced civilizations outside of our Universe by squishing huge amounts of matter in one spot.