Sorry if this doubt has been posted anytime before, but in a little view I have not seen it! (well, I have not make an exhaustive search, it could take long time!). I think this doubt must be easy to resolve: So, we have two galaxies, A and B, and they are moving... If we are at A, we can say that B is moving with respect to us with a determinate velocity, so, the time at B goes slower than at A (let me know if I'm wrong!) But the people at B can say the same: the time at A goes slower than at B! And a C galaxy could be in a way that A and B have the same velocity, and say that the time at A and at B goes equal... If there are not a "better" reference, how can we determinate the relation between "velocity" of time on each galaxy?? Thanx to all!