Ghosts, a Science.

Ghosts aren't really what we think they are. We think they're spirits of the dead, when in reality that's not the case. Ghosts are real, living, people. They aren't dead, and they aren't from our world. They live on an Earth very similar to ours, except some people who are dead here aren't dead there (and vice versa.) We can see them sometimes because occassionally, the overlap between these seperate dimensions is strong. These different versions of Earth are simply overlapping, and that is why we can see ghosts.