What theory best explains the persistent, slowly rotating pattern of spiral arms?

The density wave theory, positing a compression front sweeping through the galactic disk.

The observed spiral patterns are stable over billions of years, which contradicts the idea that they are fixed physical structures because stars orbit the galactic center at varying speeds. The accepted explanation is the density wave theory. This model describes the pattern as a slowly rotating wave of enhanced gravitational density that propagates through the disk. As interstellar gas and dust clouds encounter this compression front, they are temporarily slowed down and bunched together, much like traffic backing up at a slow-moving construction zone on a highway. The wave pattern itself persists because it is a density fluctuation, not a fixed collection of stars, allowing individual stars and gas clouds to enter the compressed region, pass through it, and then move on to the next inter-arm region.