Aerogel, was the lightest material in 2006 which was used by NASA to collect dust from a comet and is 99.9 per cent air with a density of just 1 milligram per centimetre cubed. That too was recently beaten by the metallic microlattice, which has a density 0.9 mg/cm³. This metallic material was also nearly 1,000 times thinner than a human hair.
Now there is Aerographite, which broke the previous record. With a density of less than 0.2 mg/cm³. Researchers at the Hamburg University of Technology and the University of Kiel, both in Germany, made it from a network of hollow carbon tubes grown at nano and micro scales. It is mostly empty space, though if you try to hold it in your hands, it would look like a black sponge.
Due to its ability to conduct electricity it can be a used as part of an ultra-lightweight battery.