Aren't those Vex ruins? Those time travelling robots or whatever. I thought that was the implication.
When you get to the end, the Black Garden pretty much looks like you're back on Venus anyways.
There's also a line later on where Ghost says that the Vex seem to have their shit on every planet deep down, or something to that effect. That they just had to reactivate what they buried presumably some time in the past.
The darkness isn't the aliens you're fighting. They just follow it like scavengers follow a predator. The darkness wants to kill the Traveler, couldn't care less about the humans in the way. Traveler blew itself up to push back the darkness so we could recover. The darkness use Pyramid-shaped ships.
Why do you even go to the moon? I forget. All I remember is once you go to Venus you get an actual cutscene with another character, and she sends you on this whole Vex quest. Then you stop the Vex. Then everyone celebrates and she's all "we started a new video game franchise".
It's super weird. Like playing through an episode of a procedural TV show, but as if the fifth episode of the season was episode one. The Black Garden was fun though. Game needed a whooooooooole lot more of stuff like that. Like, every two missions should've had a big set piece like that. At least there are the Strikes for that (and I guess the raids).
The Vex seemed like they could've been interesting though. The whole stuff about being outside of space and time, or was it being backwards in time? Fuck it, I dunno. Tyrion was saying some shit while I was shooting aliens.
I think it's still a decent indicator of how much of the game is left. Collectibles seems to usually fall within the last 10%, so it's not like you beat the game at 60% and the remainder is just flags and multiplayer crap.