True vacuum. Oddly, there is actually stuff in there; albeit “virtually“. Quantum field theory suggests that the void between matter and particles holds some sort of vacuum-energy that may create (only to in picoseconds neutralize) “virtual particles”.
Know that this is nearly impossible to observe, but the math and theoretical models infer this. The guess is particle-antiparticle pairs manifest then cancel out. Quantum-thermodynamic-system hypotheses have failed to find another explanation for this peculiar energy in the vacuum. Otherwise, it’s mysterious but necessary that the physical fields are there and fluxing. It’s just beyond our current comprehension why or what is exactly going on in spatial voids and non-ordinary-matter zones.
Nothing is something.