I tried making this argument 2 months ago with him in the Density buoyancy thread... He was like... NOPE, Fake News!
The world's least dense solid is a graphene aerogel with a density of just 0.16 mg/cm³; produced by a research team from the Department of Polymer Science and Engineering lab at Zhejiang University, China, headed up by Professor Gao Chao (China). The material was announced in Nature magazine on 27 February 2013.
Osmium is a chemical element with symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element, with a density of 22.59 g/cm3.