The movie takes place in the same universe of a Robot Chicken sketch. Specifically, this one. Due to Harrison Ford and Aerosmith failing to destroy the asteroid (along with the second shuttle sent after them, full of all the scientists), the Earth is doomed.
This movie takes place in the same universe as The WatchThe "Astorid" was in fact the first alien fleet, it disguied itself as an Astorid, and set up the optical illusion that the world was ending. By the time Matilda hit, after an hour or so of silence, everyone wonders what happened. They soon come to the conclusion the astorid was a dud. The Watch takes place a few years after this movie, when the aliens are done studying their enviroment, that's when "phase one" of the invasion begins.
Was it all just a dream?It could be a dream that Dodge had. Penny could be a symbolic representation of his high school love who got away. Besides, receiving updates about the end of the world and all of a sudden a Beach Boys song is playing? It's way more likely that something sad or "apocaliptically" epic (i.e. Wagner's Siegfried's Funeral March) would be playing.
Aliens sent the asteroid to destroy humanityThe asteroid arrived to hit the Earth a week ahead of predictions. It is simple math to calculate the time of arrival based upon the asteroid's speed, and there is no way the scientists could have been that far off. Therefore, it must have accelerated. There is no way an asteroid could accelerate unless acted upon by an outside force. And if it accelerated, then it would miss the Earth unless it took a different path, which also suggests outside interference. Consider also the mysterious fire destroying the space shuttle that was sent to divert the asteroid. The likely inference is that aliens sent the asteroid, probably to destroy humanity before they could reach the stars.