In this task, you are given a sentence and a question, you would be asked to create the answer which is contained in the sentence provided.
Q: Sentence: Expansion of a gas into a vacuum is irreversible because the external pressure is measurably less than the internal pressure of the gas, and no states of equilibrium exist. Question: Expansion of a gas into a vacuum is irreversible because the external pressure is measurably less than the internal pressure of the gas, and no states of what exist?
A: 
equilibrium