Mechanical Universe 15 - Conservation of Momentum - YouTube Watch the above video, I skipped to the crucial part. Provided no external forces act on a system, the system's center of mass is always in an inertial frame. If we consider the entire universe as a system, and assume that no external forces can act on the universe, then the universe has a center of mass. In other words, a center! We may even measure absolute values of relative velocities and positions from the center of mass. I am either a genius, or have become completely retarded.