Aerographene or graphene aerogel is, as of April 2020, the least dense solid known, at 160 g/m3, less than helium. It is approximately 7.5 times less dense than air.
— Massimo (@Rainmaker1973) January 9, 2024
It doesn't float because all its tiny cavities are filled with the surrounding air. If you could suck the air out… pic.twitter.com/XXur4BFzse