Question: What is the universe made of?

Stuart Atkinson answered on 11 Mar 2016:

In terms of material, astronomers believe that about 5% is visible matter (electrons, protons neutrons), 24% is so-called dark matter and the remaining 71% is dark energy. But between the stars there is very little – about 1 hydrogen atom per cubic centimetre, and very cold at 2 to 3K (about -270 deg C).