After reviewing the comments on my post about the opposite of the word dense, I began to think more about the possibility of a word like weight to describe the spectrum dense <=> rare. (Weight, of course, describes heavy <=> light.)
It occurred to me that dense is probably a latinate word while weight should come from the German, and a bit of research confirmed this hypothesis. Weight comes from the German Gewicht (adj.: wicht) and dense from the Latin—say it with me—densus, meaning not only dense but also close, crowded, and frequent (or so I hear).
Looking up density in German to find an analogue to Gewichte led me to Dichte, with the adjective form dicht. The parallel to the German words concerning weight is quite close, though I should say I don’t know the language well enough to deal with that Ge- prefix.
At any rate, the symmetry between wicht and dicht makes think that a word looking something like deight ought to be in place to describe the spectrum along which things can be relatively more dense or more rare. I think I’ll use it from now on, as clumsy as it sounds.
Incidentally, in a classic case of the English language’s notorious hodge-podgery, although light comes from German as you might expect, heavy arrives in our language by way of Old Norse.
Don't Look Now 
How about dense and vacuous as the two extremes of the rarity scale?
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Another nice idea, but I think vacuous has the same issue as your comment on my earlier post: It’s too specific to be the catch-all opposite that dense requires. Unlike heavy, dense has a wide variety of physical connotations, each with its own corresponding opposite. Vacuous is one of these, I think, but excludes to many of the others (including buoyant, your earlier suggestion).
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