Dark energy - the pressure exerted by empty space!

I would never have thought it, and even after reading it cannot begin to properly understand or appreciate this:

... dark energy is the pressure exerted by empty space. From a quantum mechanical perspective, empty space is unstable. According to statistics, photons and subatomic particles pop into the vacuum of space in a way that shows that "empty" is only an approximation: Space actually comprises a statistical soup of particles and antiparticles that are in a constant state of creation. Today scientists can demonstrate this by pumping the gases out of any empty chamber. After every atom has been pumped out, particles begin to percolate into existence in a process called vacuum fluctuation. ...

... unless the vacuum itself exerts the negative pressure observed, then the universe must otherwise be composed of as much as 70 percent dark energy.

Read more in the following Dr Dobb's article: Quantum Mechanical Theory Behind 'Dark Energy'?