space density

I have seen pictures of spiral galaxies, glistening whirlpools of stars, and I always took it for granted that (somehow) the matter in a spiral galaxy was concentrated in the very visible arms. Not so:

The spiral arms are delineated by a high space density of particularly luminous stars and luminous interstellar clouds. Elsewhere in the disc the space density of the stars and interstellar clouds is no less; it is just that they are not as bright.

I suppose someone has a guess why matter separates into luminous matter and less luminous matter (which is not “dark matter,” right?).

source: Pluto: sentinel of the outer solar system by Barrie W. Jones