Consider a matter and antimatter pair having the same mass and one of the two has been accelerated by an external force or supplied with some initial kinetic energy(After big bang least one of the many formed matter and antimatter pairs may have different kinetic energies).According to relativity mass varies with speed even if the variation may is minute there will definitely be a mass variation for two objects having same rest mass and moving with different velocities.So the mass of antimatter and matter are different.Or does the rest mass of the two have to be the same?