Warning: This list contains spoilers for the entire Alien franchise.
Ever since the first chestburster exploded out of Kane and onto the big screen, fans of the Alien franchise have tried desperately to work out the mysteries of Xenomorph mythology. The six features in the Alien saga (eight if you include the less-than-stellar Alien vs. Predator films) have each expanded the lore in new and unexpected ways. Even with 40 years' worth of films to draw from, a lot of nagging questions about the franchise remain. Why did the Engineers create the Xenomorphs? Why is Ripley so important? And what's with David's flute fascination?
We may never learn the answers to these questions, but that hasn't stopped the internet from trying. Dozens of fan theories exist on the web that aim to tie the films together in a more cohesive way. Many are completely ridiculous, but others genuinely make the films better. Only you can choose which fan theories are essential to the franchise - and which should be thrown out the airlock for good.
The Black Goo Is Xenomorph DNAPhoto: Prometheus/20th Century Fox
This theory from Redditor /u/HeronSun suggests the Engineers were not responsible for the creation of the Xenomorphs. Instead, they posit the black goo was a result of the Engineers trying to distill the essence of the Xenomorph into its pure, raw form.
Through experimentation with the naturally occurring Xenomorphs, they produced a substance that rapidly transforms a subject into a crude Xenomorph and hoped to use it as a weapon.809277Does this make sense?
Ripley Is Genetically Predisposed To Harboring A Xenomorph QueenPhoto: Alien 3/20th Century Fox
Poor Ripley. That woman has been through a lot, and Redditor /u/clutzyninja has an idea as to why she specifically can't get far enough away from the Xenomorphs. They believe the Weyland-Yutani corporation knew about the Xenomorph when they sent the Nostromo to investigate (an overlapping theory). Not only that, they suspect Weyland-Yutani sent that specific ship because Ripley was aboard.
A company of that size and influence would almost certainly have genetic information about their crew members, so the theory is Weyland-Yutani found something in Ripley's DNA suggesting she would make the perfect host for a Xenomorph Queen. This is further backed up in Alien 3 when Ripley finds out what's infected her is not just any old Xeno.
If Ripley truly is uniquely suited to host a Queen, it could explain why Weyland-Yutani seems to do everything in their power to get Ripley implanted.980350Does this make sense?
The Engineers Created The Black Goo To Wipe Out HumanityPhoto: Prometheus/20th Century Fox
Redditor /u/HeronSun shares another theory directly connected to the belief that the Engineers invented the black goo. This part of their theory suggests the Engineers distilled the black goo from the Xenomorphs to wipe out humanity. The reasoning here is tied to humanity's eternal quest for power and how humans in the series constantly want to weaponize the Xenomorphs.
The Engineers were concerned that if humanity expanded throughout the galaxy, they would become a conquering species and lay waste to thousands of worlds.854304Does this make sense?
David Didn't Kill Walter - He Became Walter
The big twist (that most saw coming) at the end of Alien: Covenant is that David murdered and replaced his identical android brother, Walter. Redditor /u/chaddified doesn't believe it's that simple, though. David and Walter have slight physical differences, but when David's identity is revealed in the end, he still possesses some of the physical traits of Walter.
Instead they suggest David didn't simply kill Walter, but he somehow transferred his consciousness into Walter's body. This could have taken place a number of times, like in the scene where David kisses Walter. This would also explain away those minor differences and fits the theme of forced body-takeovers beloved by the Alien franchise.764281Does this make sense?