Why is the speed of light a constant? Can anyone tell me if my theory is ridiculous?

If our universe is expanding then 'could' it be safe to ume that there are different speeds involved. Is it possible that the speed of light for earth and the milky way galaxy is faster than say - on the other side of the universe?... if that's the case, then can we travel faster than the speed of light elsewhere in the universe relative to our own... hence, our spaceships can travel x here and x+a over there... (why is it constant...what stops this from being true?)