en.wikipedia.org/wiki/Empirical_formula
In chemistry, the empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound
In this comic, the “compound” is the entire observable universe. So it’s the simplest whole number ratio of every atom in the observable universe
A more in depth explanation for this comic (and all other xkcd comics) is here: www.explainxkcd.com/…/3200:_Chemical_Formula
plactagonic@sopuli.xyz 1 month ago
It just lists the elements in a form of chemical formula with their estimated quantity in universe.