The "fifth element" is actually Spiral Power.The power of life, driven by love... able to overcome anything... Leelo defeats the big bad evil incarnate with a carebear stare, complete with countdown.
This isn't the first time Mr. Shadow got defeated this wayAt the end of the climax, we are left with a shot of Mr. Shadow's dead body floating above the Earth, with the Moon also visible. Maybe the origin of the Moon is just another of Mr. Shadow's attacks? Or even another baddie like him?
That plastic...thingy on Zorg's head is a cybernetic implantHe's rich and I want to believe there has to be a reason for anyone to have that on their head.
The Mondoshawan are Starfish Aliens.The Mondos wear protective suits, even when piloting their ships. Their actual form is completely inhuman, probably something unfit to any environment but their own homeworld. Maybe they really look like huge jellyfish or octopi. Their exosuits are actually the closest they can get to looking humanoid.
The Diva is actually not of an alien race, but is a genetically-engineered marketing gimmick for Listerine.I mean, come on. Her blood looks just like the stuff.
This film takes place in the Stargate universe.Seriously. The Egyptian stuff. The ships look like they could be using the same tech as the Promethius. The clothes of the soldiers does kind of look like that clothing worn by Quinn's folks.
The Mangalores were created to be warriors to fight on Earth.This is why they are exceptionally good at disguising themselves as humans. They were meant to be able to infiltrate human armies and groups. However, they Turned Against Their Masters. This is why the airport on Earth has a machine that can detect them.
The black slime that comes out of General Staedert and Zorg is their inner evil liquefied.It is the "badness" in them being made physical and drawn forth from their bodies towards the source of all the universe's evil - the Dark Planet. This also explains why Zorg had more of it than Staedert: we've all done something bad in our lives, and Staedert, a career military general of high rank, likely did some morally questionable things in his time (an idea supported by how readily he seemed to want to "shoot first" when the Dark Planet manifests - evil begets evil, as Cornelius says), but still doesn't leak as much "evil" as Zorg does despite being right in the Dark Planet's presence.