Aerogels are composed of 90-99.8% air

 Aerogel is a low-density solid-state material derived from gel in which the liquid component of the gel has been replaced with gas. The result is an extremely low density solid with several remarkable properties, most notably its effectiveness as an insulator. It is nicknamed frozen smoke, solid smoke or blue smoke due to its semi-transparent nature and the way light scatters in the material; however, it feels like extruded polystyrene to the touch. [WikiPedia]