In the video, Thermal Protection System Engineer Betsy Congdon says it's 97% "air."
I can't say whether it's actually air, or she's simplifying things for the general public or not.
She also says twice that "water" is used in the radiators. But I'd have to believe that NASA's using something that absorbs/dissipates heat a little more efficiently. Perhaps whatever it is will end up in desktop gaming rig cooling systems eventually.