C'mon it's not that bad. Dont take visual effects too seriously, they are bad and that's it. If you take a closer look, the entire movie is enveloped in darkness until the end... but when the explosion kicks in, sun shines throughout the screen and everything gets so bright! They are even heading to Canada! Answer couldn't be more simpler: It was always a dream of John, we don't know how it all started we just saw a glimpse of this apocalyptic "universe".