I'm referring to densities of materials, not the number of particles per unit volume.
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Density is a property of matter describing how closely packed together its molecules are (dictionary.com). It is essentially the same use as a population description where many people packed closely together is considered a dense population.
Because everything has a density, but different densities are relative, the closest thing to the opposite of Dense you can get is either an absence of matter all together or Not Dense, or Less Dense. I'm sorry I can't offer anything better, but in all my years learning and teaching chemistry, I don't believe I've ever seen or heard an actual opposite for the term.