OK, I am not a physics expert by any stretch, so my suppositions here may be totally wrong but here goes. In fictional time travel a person can go into the past, but that would entail removing matter from the present and putting extra matter into the past, which would create a discontinuity over time in the amount of matter in the universe.
OK, that is fiction and nobody cares about that minor detail. But in actual physcis I believe there are theoretical models of time travelling particles, etc. Again I may be completely off base here. But if there is, what happens to the conservation of mass in those cases?
Please be gentle if this is a dumb question.