Question
When a person "POPS or Cracks" their back. What is happening exactly? Is the Annulus filled with air or water?
Is there air between your spine?

Answer
There is Nitrogen gas dissolved in solution in the joint space.  When the back "cracks", the pressure in the joint has been changed, allowing the gas to come out of solution.  The Disc itself is centered with what is called the Nucleus Pulposus.  It has the consistency of Jello.  And, no, there is no air in the spine :)