Even in completely empty space, with no external fields present, There's still some amount of non-zero field energy existing in any region.
If quantum fields are everywhere, according to Heisenberg's uncertainty principle, there will be an inherently uncertain amount of energy present in that region over any chosen duration of time.
The shorter the duration we observe, the greater the uncertainty in the energy amount.
Considering all possible quantum states, we can envision fluctuating fields and particle-antiparticle pairs that sporadically appear and disappear due to the Universe's various quantum forces.