All of the material in the galaxy orbits the galactic center. If the galaxy were a rigid body, all of the gas and dust would orbit with the same period. However, material closer to the galactic center orbits with a shorter period than material farther out. This is not an unusual situation. After all, the planets in the Solar System exhibit the same behavior: Mercury takes less time to orbit the Sun than does the Earth, and so on. When the orbital period depends on the distance from the center, we say that the material is exhibiting differential rotation.
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