Let me start of by saying that I'm no lore nerd and all of this could have been went over by somebody else but Im yet to see a thread or video that has thought of this, So if your going to hate and instead of kindly telling me then -blam!- you :D
So basically I thought of this theory while patroling and kicking alien ass quite some time ago, So while headshotting Cabal and Fallen enemies their heads will sort of implode and a dark soul-like mist will rush out from their heads. After this I got curious and thought to myself its no coincidence that the two "known to be evil" races didn't have this darkness rush out of them when killed but the two races that have been thought of to be possessed or controlled do! So after some digging around I found this image...
[spoiler]http://www.destinyoverwatch.com/attachments/destiny-races-png.27/[/spoiler]
This is an Image of 5 destiny races. The first four are obviously Vex, Fallen, Hive and Cabal but the 5th one is not very obvious or known (To me at least ;P) But I do have to say that Im really drawn to the fact that the dark smoke-like part of the drawing/painting looks an awful lot like the soul that spews from the heads of dead or should I say 'cleansed' Fallen and Cabal enemies.
Basically I think that The darkness is a being/race and possesses victims of the traveller, this theory explains the fifth race we see in that concept art and the random animation that the Fallen and cabal share when shot in the head. To me this thory seemed quite obvious but as said im no lore nerd and this could be really stupid. Feel free to comment what you think and if you agree or disagree with me :)
Also if anyone could take any pics of the dark smoke/soul stuff that you see when headshotting these enemies so I can apply them to the OP for all to see that would be great!
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Well if this game had any resemblance of a story we would probably know some of these things. Of course the shooting and blowing shit up makes up for it!