Aerogel

An Aerogel is a super-porous silicon-based substance that is over 99.8% air. Aerogels are currently being used by NASA to catch micrometeorites, as well as for insulation.

Since aerogels can possibly (rarely) occur in nature, there has been some speculation that there are aerogel-based organisms living in the higher reaches of Earth’s atmosphere. These organisms would be almost invisible, nearly weightless, and would almost never reach the surface without dissolving.