Does that energy not come from the temperature of the gas? That is, isn't this why gas cools as it expands? The gas takes up more volume for the same mass, so has lower average thermal energy. Eventually the gas stops expanding because it can lose no more thermal energy to the surroundings.

I also can't help but think of buoyancy when we talk about displacement. I guess that's a red herring, an object immersed in water in space would neither sink nor float. But then again the object still experiences a force from the surface tension of the water. Is there an analogy here?