Wow, I never thought of the background being nitrogen, it makes perfect sense though. Great thought!
I had that thought too, but it doesn't really work since you can set the ambient temperature of the medium to 0K and not have any nitrogen precipitate out.
Because hydrogen and invisigen would explode. Also, seeing as LN2 (liquid nitrogen) dissipates, it must be nitrogen. It just works, roll with it :P
TL;DR it can't be oxygen or hydrogen because science