From classical theory, a spiral structure of galaxies caused by gravitation instability in stellar disk. Most unstable mode is one arm, but there is problem with the center mass, so two arm structure is more common. But from real observation, even for very good two arm galaxies, arms are not symmetrical and one arm is more bright and massive then other. This can be explaining that real system has two components: stellar disk + dark matter (or black hole, or dark energy). In this case a shift of mass in stellar disk balances a shift of mass in second component.

If somebody tries to estimate this shift of mass and explain it or black hole, dark matter or dark energy? What is structure of dark matter in spiral galaxies?