None. I thought. Methane is also flammable and a vacuum is not a gas and not currently possible. But wiki says Ammonia (Hard to combust)- https://en.wikipedia

None. I thought. Methane is also flammable and a vacuum is not a gas and not currently possible. But wiki says Ammonia (Hard to combust)- https://en.wikipedia.org/wiki/Lighter_than_air https://en.wikipedia.org/wiki/Lighter_than_air Off planet you obviously have other options. But a complex molecule might work I assume?