DISCLAIMER: I am not a physicist, so any of the following may not be entirely correct:
Vacuum energy is a result of the Heisneburg Uncertainty Principle. According to this principle, the energy of the vacuum can assume a nonzero level for some short amount of time, the amount of time depending on the amount by which the energy differs from zero, the more energy, the shorter the time. The end result is that a particle-antiparticle pair can form so long as they annihilate each other within some short time span. (I still fail to understand why we don't see gamma rays formed by this process. I understand that the energy must return to zero, but what happens to the gamma rays formed in a p-ap annihilation?)
As far as causality goes, it may seem counterintuitive, but quantum mechanics has shown that the idea of causality may not be particularly useful in describing the microscopic world. For example, in a sample of radioactive material, we can predict pretty precisely how many decay events will occur in a given time. However we cannot predict the amount of time that will pass before a GIVEN radioactive atom will decay. What is the cause that causes that atom to decay at the time it does and not some earlier or later time? There doesn't appear to be any causality behind such microscopic events.
As far as whether or not space is really a "something" and whether the curvature of space can contain energy, general relativity sure seems to me to treat space-time as a thing. It speaks of the curvature of space-time and postulates that gravity is caused by this curvature. Since there is energy associated with gravity, there is also energy associated with curvature of space-time. If I understand it correctly, that's one of the problems with a quantum theory of gravity. It would have gravitational interactions occurring via exchange of particles known as gravitons. However, the graviton, which would be a quantum of space curvature, would have an energy associated with it. This energy would tend to cause further curvature, according to GR. This would result in more gravitons and further curvature. This leads to the prediction of infinite energies from a quantum gravitational theory, which clearly is problematic for a theory in physics. Hopefully this helps some, and I of course am anxiously waiting for some of the resident physics experts to tear what I've said apart.
Thank you for your reply.
So, does the vacuum energy consist entirely in the particle-antiparticle pairs? In other words, is the energy of a given volume of vacuum zero when there are no pairs present in that volume? Or are the paricle-antiparticle pairs the result of a non-zero energy state which subsequently manifests itself as virtual particles? Can the energy volume of a vacuum be non-zero when no virtual particles are present?