Ultimately, at small-enough dimensions, physical effects
lose the continuity that our crude senses give them. Movie frames
flashed before our eyes at twenty-four frames per second seem
continuous, yet the quantum nature of film is well established.
The mind cannot comprehend a substance that has simultaneous
natures of both particle and wave. It is not like mixing black and
white, or reducing the squares of a vast checkerboard until we
cannot distinguish the pixels but see only a field of gray.
Werner Heisenberg taught that the probabilty of being in one
state or the other was not resolved until we measure it. Until it
hits the eye, light has a virtual existence as both particle and
wave. Could the same be true of space? If virtual particles
inevitably exist in the absence of particles, maybe it is the dark
energy that can explain the expansion of the universe.
See also in The book of science:
Readings on wikipedia: