All velocities can only be measured relative to something. To answer the question, we can only say with any certainty that one object is moving at a speed of x relative to another object. One galaxy to another, an asteroid to a planet, one car to another car.
There is no "true speed", all speeds are relative.

... I guess that means that speed isn't a fundamental quantity of physics!!?? *gasp*

I hope that helps or at least confuses the subject a little more.

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