Okay, based on the replies i got, a few people said that it has to do somewhere in the middle of ME3. Sorry, but i don't see how that timeline works for 3 reasons:
1) For any species to build arks the size of citadel, it will take a huge amount of resources and especially, time to build that. I highly doubt Alliance can pull this off alone without the help of other species. They took the help of every major species to build one Crucible, but 3 arks, alone? I don't think so. And about the time, it took 6 months for the Reapers to start the attack, again, i highly doubt they would've finished building those arks in 6 months, i don't care how advanced their tech is, it's just not possible. And if the Alliance did somehow pull this off, then why would the Alliance officials ask Shepard on what to do with the Reapers in the beginning of ME3? Why didn't they just immediately start evacuating the humans before the Reapers reached Earth? What's the point of making a Crucible? What's the point of ME3 anyway then? They could've just evacuated everyone, would've saved a whole lot of people that way. Especially Shepard.
2) Nobody, except Hackett, Anderson and Shepard's squadmates, gave a damn about what Shepard had to say. We all knew that when Shepard went to the Council and they refused to help them. Without the Council's support and it's resources, i doubt just big arks would've been possible.
3) If they did manage to build those arks, there's no way, it happened before the end of ME3. Reapers were already attacking the Earth, and the Reapers were guarding the Earth from any reinforcements to come. In the trailer, it looked like a casual transferring of humans to the ark, i didn't see any sign of fight.
4) The Dark Matter plot ending was supposed to be A) Take the necessary sacrifice in becoming the human reaper, in which humanity will have to sacrifice for the sake of the organics. B ) Screw the Reapers, we'll figure out a way to survive this. I don't know about you lot, but if this had happened, it would've been easy for Bioware to transition from ME3 to Andromeda, instead of trying to dodge the 'ending' bullet.
Like i said, it's just a theory i had about the Dark Matter, but if Bioware literally goes ahead with the whole 'let's spend billions of credits and God knows how many resources into building gigantic arks to just to travel in an unknown galaxy', then i'm afraid they're already starting things in a bad way. Mass Effect has never been simple, there were always these situations which were huge enough for species to be in a disarray. ME1 had Saren, ME2 had Collectors, ME3 had Reapers. So a simple, let's explore a galaxy we have no idea about instead of chilling in Earth, without a big enough reason as to why the big change, is just weird.