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