Single word for "less mass per unit volume" (the complement of "dense")

In statistical physics, density is an important concept, and I have not encountered it used in any way to suggest it has an antonym, sorry to say.

So, I would have to conclude that there is no "science word" for less dense in a general sense. Rarified only applies to gases, and often only to air pressure when discussing the propagation of sound waves.

The intensive property is density and when a given material has a larger or smaller value for density than some other material, we just use "more" or "less" as appropriate descriptors.

Depending on your context, you could possibly use the word diffuse, which has the general meaning of things that are more spread apart, but I would not consider it an exact antonym to the scientific usage of dense.

Try sparser or scanter........