I'm not sure what's in there. Depending on the amount of light and the angle viewed at, it sometimes looks like air, it sometimes looks like dirt, and it sometimes can't be seen at all. The other crummy thing is that there are stretch lines in various places. Rik said the kit comes small and has to be stretched into place, so there's nothing that can be done about that.
Anyway, I hope I'm worrying needlessly. He seemed like a decent and honest guy, but I'm worrying anyway just because I shelled out big bucks and it looks chintzy with the bubbles all over the place.