I've found its best not to think about it too much.
Wildest theory I heard was some weird one that theres two universes running parallel to each other, one going forward, one going backwards, but we might be the universe thats playing in reverse, but we just don't realise it, as which ever universe you were in would feel like the one playing the "correct" way. The "mirror universe theory" or something, read it a while back.