My researches show that the average densiity of the universe is about one atom per cubic meter.
That isn't very much, is it? I mean, there's an awful lot of atoms in a galaxy and there are an awful lot of galaxies. In that case there must be awful awful lot of very empty space.
Then I got to thinking, how do you measure the density of the universe anyway?
Oh yes. First you measure its volume then you weigh it. Simples.