Where did the energy that caused the Big Bang originate?
There are many theories that have been devised to attempt to explain the origin of the universe. The Big Bang simply denotes the instant at which some primordial, currently unknown, type of energy was instantly converted through a large, universal inflation to create the elements and particles that we know to be a part of the standard model.
What that primordial energy was or what made it "unstable" is not known. You could look into research done on quantum fluctuations or string theory and parallel universe collisions if you want to try and decipher the current working theories.
If you found a way to prove a theory experimentally for the state of the universe before the Big Bang you would be on your way to a Nobel prize I'm sure :)