The arms are regions of active star formation, comprised of hydrogen gas and dust. Their masses range from 10s of millions to several billion Solar masses. Acronym for a Weakly Interacting Massive Particle, a possible form for cold dark matter . Key Concepts 1. Our Galaxy, appears to be a typical spiral galaxy, and a twin to Messier 31.
Many nearby galaxies are likely to possess billion Solar mass black holes.
Books Malin, David.