What is the lightest thing in the world?

The lightest solid material in the world is aerogel, which is composed of 99.8% air and has a density as low as 3 milligrams per cubic centimeter. Aerogel is sometimes referred to as "frozen smoke" or "solid air" due to its extremely low density and translucent appearance. Despite its light weight, aerogel has incredible insulating properties, making it useful in a wide range of applications, including in spacecraft, as a building material, and as a thermal insulator.