What is the lightest and lowest-density solid known?
Aerogel is the lightest and lowest-density solid known to exist to date. It is really a mixture of glass and quarts and a ethanol. So waht? well it is a WONDERFUL material. It is one of the best known insulator. transparent and blue in color. New technological advance should soon permit clear/transparent aerogel.
Follow the first link below to find out how the stuff is made.
http://www.cae.wisc.edu/~aerogel/aboutaerogel.html
More interesting links on Aerogel:
http://science.nasa.gov/newhome/help/tutorials/housefuture.htm
http://www.mkt-intl.com/aerogels/index.html