<Well... here goes Nothing.
What is the [b]Darkness[/b]?
The Darkness is the ancient enemy of the Traveler, even before the Hive came to be. It was once imprisoned beneath the oceans of Fundament, held there by the Traveler and the Ammonites and the Leviathan (not to be confused with Calus's Grand Leviathan). Since the Hive rose to power, the Darkness was released and has been terrorizing the system for quite a while.
One of the first beings to notice the Darkness during the Collapse was Rasputin, the Warmind. He says it inhabits its petitioners, like the Traveler does with humanity and the Guardians. He also calls it many names. IT, the Flower-Eater, and the Queen of Final Shapes. Mainly IT. Why does he call the Darkness these things? Well Flower-Eater is one of the most obvious. The Traveler seeds worlds, the Darkness destroys it.
Why IT? Because even to Rasputin it can't be quite comprehended. So let's prove Rasputin wrong... maybe.
Why the Queen of Final Shapes? For one thing, the Final Shape is the Darkness' goal. Any lifeform that can't defend itself should be eradicated. It wants to rule the universe. The Final Shape is its rule.
Why Queen? Toland ranted on, trying to explain the Darkness. He brings up a thing to relate it to. Imagine three Queens he says. The one that relates to the Darkness is the third, the Queen of the Country of Armies, the one that raises an Army and Conquers everything. Not sure if this is really related to each other, bit of a stretch, but it's not the only thing...
Look at this picture:
https://cdna.artstation.com/p/assets/images/images/007/415/032/large/mark-kolobaev-bungie-concept-artstation.jpg?1505983337
This is a [b]supposed[/b] concept for the Tetrahedral Fleet... up-close. Society and whatnot. The Tetrahedral Ships are the cover for this post. This is the enemy of the Traveler.
The next piece of evidence for "Queen" comes from this. Notice how all 4 statues are female? The top 2 seem a lot more "royal" than the bottom two as well.
Also a leaked quote from Lakshmi-2 says "the hunt for Tenebra has just begun". Tenebra is a feminine Latin word for Darkness.
Moving on.
Calus apparently saw the Darkness. He described it as the Nothing or the Void that was stronger than him... and he also says it's stronger than the Light but that's what everyone says.
Pujari believes that the Darkness is a physical and moral presence. We know it's physical, but moral? An actualization of evil? That's not what IT believes, but IT is also trying to kill all life so...
Acataleptic Clause says we are unable to understand the Darkness. That explains why Rasputin just calls the Darkness "IT".
The Monist Position says the Darkness is a technologically advanced force, to the point of post-singularity intelligence. Considering it's a massive fleet of ships numbering in the dozens and has the ability to grant powers to its followers, I'm not denying it. The problem with this one is that it comes with the theory that the universe is a simulation. So take this all as you will.
Ulan-Tan's Thesis, arguably the most popular theory, is that the Darkness is a necessary symmetry to the Traveler. The biggest piece of evidence for this is when the Traveler woke up after the Red War, so did the Darkness.
Saint-14 says the Darkness is an invading alien armada. One that was made because they were straight up REJECTED by the Traveler for their sins. Something to take into account here is that, according to the Perfect Paradox theory, we have yet to meet Saint-14. It's very possible that we meet him shortly before the arrival of or the aftermath of the Darkness. I think we should take him seriously on this matter.
Also the fact that he's kinda right. It IS an alien armada... BUT... I refer you back to picture 2. Look in the middle there. A ball covered in markings mostly consisting of circles. Traveler? If that's the case, then it can be inferred that they were rejected.
What we know about the Darkness right now through the in-game cutscenes and whatnot is that it's a fleet of Tetrahedral Ships, with one bigass one in the middle of it all.
What about its inhabitants? There's picture two, then there's concept art for the "fifth race" that was made ages ago, and then there's Cayde's Journal. Picture 2 depicts them as VERY human-like... almost too human-like. The concept art doesn't help as it's simply a stick figure with a white head. It could theoretically be much more than that. Then again, you have to remember in the original storyline where this art was made had US as the "Darkness".
Cayde's Journal depicts 2 shadow creatures. They could very well be some soldiers of the Darkness. One is almost bestial looking and the other is heavily armored. Considering that all we've seen of them and their possible society is like one picture, these two could be a part of it. Otherwise, it could be a hallucination probably caused by the severe pain by the "boiling shadow".
Now here we go... directly from the Hive.
Records on the Dreadnaught show that the Darkness is an embodiment of the universal struggle for existence. The Traveler does the "peaceful" things and the Darkness annihilates anything incapable of defending themselves.
So it may not JUST be an angry armada.
Remember the Heart of the Black Garden? Well, there are two theories on what it could be. One is a worm captured by Vex Mind Quria during the Incursion on Oryx's Ascendant Realm. The other theory is that it's a shard of the Darkness. I don't know about you but I don't see any of the ships changing shape like that.
Assuming theory 2 is correct, we have a conclusion (or at least until we get more shit).
The Darkness is a primordial entity, like the Traveler. Also like the Traveler, it can give its power to anyone it wants. It also has the ability to directly corrupt a being and take control (Taken) or even gift them to another entity. It has an end goal of conquering all that can't bear its might. It also changes shape, which might be why the Hive call it the Formless One. Apparently it can appear as simple black tendrils.
Given the Taken, it apparently has its own "Dark Dimension" that it uses to brainwash them.
It destroys anything in its path, especially things brought up by the Traveler. However it is a symmetry to the Traveler, suggesting that if it died, then the Darkness would die with it.
It is hard to comprehend to the average joe and even Warminds. This means that it keeps itself hidden very well, or just destroys anything capable of understanding it.
Right now it's with an armada of Tetrahedral Ships. Those in the ships are humanoid entities that have a female-oriented culture and once worshiped the Traveler... until it rejected them. The ships themselves are capable of destroying entire worlds and rendering any survivors helpless. They are active when there is Light coming from the Traveler, meaning they deactivated when the Traveler went dormant.
And now they're on their way.
We're -blam!-ed.
We're so -blam!-ed.>