Quantum Mechanics includes the concept of a False Vacuum:
vacuum in Quantum mechanics indicates the lowest possible state
of energy, and a false vacuum is a state of energy that "seems"
lowest in that there is no lower state within "interaction" reach.
If the false vacuum has the opportunity to interact with a True Vacuum,
the false vacuum exhibits a negative pressure in relation to
the true vacuum, causing it to rapidly shrink.
Some theories of Inflation, including the original one,
suppose it to be such an interaction involving a
false vacuum present just after the Big Bang.