My lecture notes state that if gas particles sit in an insulating container, with a partition separating the gas from vacuum is removed, no change in internal energy is done, and it is stated that it is because the thing separating the gas is vacuum. I'm confused as to why this implies that the gas particles do no work on expansion, and I think this is touching on a general confusion of mine.
Without thinking about maths, why is it that a gas expanding into a vacuum does no work, but expanding into something that isn't a vacuum does do work?