Matter cannot be created or destroyed through chemical reaction, but can be through nuclear reaction. That's what E=MCsquared means, energy=mass x the speed of light squared. That is the basis of a nuclear explosion. A great expulsion of energy would be required to send one forward in time, and a great gathering for you to pop out on the other end, or perhaps just something of equal mass from the future brought back. That is only possible in the case of an artificial wormhole though. I don't believe that parallel universes can exist in a similar state to ours, the chances are just too slim for two waves of existence to exist with the exact same set of physical law. In the end, I guess it wouldn't really matter if a specific point in time was heavier than another, as long as there exists a point in which equilibrium is reached again. In the end, the universe just have to the same total amount of mass and energy when the cycle starts again. This also is a possible explanation for Mole. When you go back in time, you already exist. The various compounds and nutrients that your body dissolved into are in the soil, and they don't go back again. It's like you were born a man with memories in 1666, died and became other particles, were born a baby in 1983 and left no particles in 2004.