> Mathematically, the 80/20 rule is roughly described by a power law distribution (also known as a Pareto distribution) for a particular set of parameters. Many natural phenomena distribute according to power law statistics. It is an adage of business management that "80% of sales come from 20% of clients." [1][2]
Though really the issue with the comparison is that it implies that 80% (or ~83.3%) of the <something> is correlated to the light matter, and the other 20% (or ~16.7%) of the <something> is correlated to the dark matter. Maybe eventually we'll learn enough about dark matter and discover that, yeah actually, 1/6 of the life, or unobtanium, or whatever, is actually in the dark matter. But for now, I agree that the Pareto principle seems pretty irrelevant.