This is a great video demonstrating what black holes are and where are the supermassive black holes, remnants of super-massive stars that have died (video compliments of Spacerip).
Super-massive black holes form when massive stars end their lives, exhausting their fuel. When fusion is no longer counter-balancing their massive gravity, super-massive stars collapse under their own gravity.
The collapse is so intense and the force of gravity is so severe, the stars implode to create a black hole. If the collapse is large enough, it can form a super-massive black hole.
These super-massive black holes exist at the center of every galaxy, including our own. They are simply the most massive and most dangerous objects in the universe.
Maddalena Environmental Inc.