We need to use different wavelengths of light, from radio to gamma-rays, in order to understand the whole picture of the Milky Way. The disk has younger, second generation stars, and has spiral arms. The further out from the center of the galaxy a star or gas cloud orbits, the more mass is contained within that orbit. The mass in the galaxy is distributed more evenly than, for example, in the solar system. There is more mass in the galaxy than we can see.
Spiral arms are due to density waves traveling through the galaxy.