The plot?[SPOILERS!!!]

they left that one pretty much wide open. After just seeing the movie, i am pretty sure it left "fill in the blank" areas on purpose. To write something about the engineers home world and who they were fighting would be extremely difficult to show without another 2 hours at least.

Well if humans were made from their DNA, wouldnt it be safe to say that theres a chance we would think like them? If they had a population large enough to travel to different planets then they would probably have governments. Which leads me to believe, maybe they were fighting a galactic war against their own species! Just like we wage war amonst other humans over sections of Earth, maybe their in a war against themselves over a part of the universe. They clearly had a larger portion of the universe mapped out. Also what does this mean about the Predators? Certainly they wernt created by the Engineers. But if the predators are seen as hunting the aliens for sport does that mean that they are unaware that they were engineered by another species? If they did then wouldnt they be hunting the engineers and not the humans?

This is what I think (taken from my post: http://www.prometheus-movie.com/community/forums/topic/7102) - There may be two types of Engineers. One that we saw at the beginning of the movie arriving in the saucer type ship (I’m calling them the ‘Creator Engineers’) and the others that were found on the planet Prometheus flew too (the ‘Killer Engineers’ ) - I think that the Engineers were having a civil war and the Creator Engineers were losing the war. They needed “soldiers” and therefore created life on earth and left directions to the planet where the Killer Engineers were creating their weapons (black goo). The reason the directions from the images found by Shaw led to this planet, could be due to the planet being the source for the black goo, and the Creator Engineers wanted their soldiers (i.e. humans) to destroy it all. - When the sleeping engineer was awoken by David, he thought that the “soldiers” had arrived to kill/stop them and went about killing the threat. And the reason he wanted to take the ship to earth was to eliminate (or use humans to breed xenomorphs using the goo, for their own purposes) the army the Creator Engineers had created.