Definition of physical vacuum
This refers to the absence of any physical particles, but there is no space for nothing. And suppose you drive away the gas in a space, you will find that from time to time there are basic particles appearing in the vacuum and disappearing, out of nothing. The physical vacuum is actually a sea of energy that constantly fluctuates. When the energy reaches the peak, the energy is converted into a pair of positive and negative basic particles. When the energy reaches the trough, the pair of positive and negative basic particles are mutually annihilated and converted into energy.