Say you were to attempt to travel between multiple universes. Perhaps by traveling back in, and subsequently altering time, thereby causing you to exist in a universe parallel to your origin (as is stated in one theory). Now, it is generally accepted as a law that matter cannot be created or destroyed. Therefore, the total amount of matter in the universe must be a constant. But if you were to travel between multiverses, you would be reducing the amount of matter in your origin universe and increasing it in your destination universe. My question is, how would this be possible? Does the law of conservation consider all possibly existing universes, therefore negating any issues caused by travel between them?