The reason the galaxy does not spin around me (relative to me) at a trillion times the speed of light when I do a pirouette is the inertial frame of reference to which I am confined along with the rest of humanity.
Where does this frame of reference end?
To the best of my limited understanding, gravity doesn't just stop at some point. Anything that has mass has gravity; gravitational waves (or whatever) extend from the mass spherically. They weaken as they get further away from the source, they get pushed around by other, more powerful, gravity waves, they become progressively weaker as the distance increases, but do they ever vanish completely, i.e. get reduced to absolute zero? Or do they become weaker and weaker forever, without zeroing out? And if so, where does a frame of reference end?