The parallel universe theory states that an infinite number of universes outside of our perception could exist... while it's debatable as to how practical this theory really is, considering how impossible it is to prove its accuracy, it seems like it could very well be a big part of UNDERTALE! After all, why do monsters recognise us after resetting the game? Why does the genocide route forever haunt the game after its completion? Perhaps all the timelines are happening simultaneously, waiting to converge...