Permittivity can be described as the opposition of an electric field (and accordingly flux) by a medium through which the field passes. I understand this opposition is generally produced by dipole moments of the atoms within the medium, but in the absence of these atoms and thus the dipole moments (as in free space), what causes the opposition?
In other words, what opposes an electric field in a vacuum and gives rise to $\epsilon_0$, the permittivity of free space?
I will try relating my answer to quantum degeneracy pressure in electron-gas model of solids. In that case , the pressure is a complete consequence of the symmetrization requirement of the fermions , and it is not any physical "pressure" as such. Similarly , free space has Quantum field fluctuations and the electromagnetic field in free space is quantized (from quantum electrodynamics). And so, this quantization requirement (analogous to symmetrization requirement) results in the physical "opposition" (analogous to pressure) like effect in this electromagnetic wave propagation (Electron-gas solid model) at classical level .
