You need a plot device called an ASB. http://en.wikipedia.org/wiki/Alien_space_bats It means there is a horizon point when the setting of your story gets so foriegn, anything you can come up with is as dumb as "alien space bats did it". Sci fi writers in particular have this problem, and they tend to compensate by setting the story in the far future or other planets. I have smoken too many pots