An idea like this works best as a member of the world not tied into the solution and not knowing the actual cause, just the bare basics.
Trying to explain or justify much of these settings is usually pretty detrimental to the plot so it's not worth the effort to actually explain that away, there's always people against it, always people disagreeing with you.
Take me for instance nitpicking the volcano idea already thrown out there, because I like geography so I'd barge in to say something like a geographical incident like a volcano (let's say the Yellowstone supervolcano) would surely blot out the sun for some time, but everyone would know, everyone would know it's temporary, and everyone would be more concerned with dealing with the heavy ashfall, the dwindling ecosystem now plants can't get light for maybe 2 or 3 years before it clears on its own.
And really it's the same with almost any justification, especially in RP where it works best when you don't know but the players oh so often practically demand answers. Nah, just don't give them.
Whoops now we've got our second issue. What's actually interesting about this beyond the spooky darkness? People will adapt, but really the main effect of this is the world will be colder, it'll have dwindling food, it'll be harsher, but by no means unsurvivable, it's just clouds as far as I understand, Let's assume it's global. Okay, this is a catastrophe, it's mysterious ever-present black clouds, is it pollution?
Do people have any real motive to fix it, is anything stopping them? Why would there be? Why are they activists? Would this not literally by a worldwide goal? So why is it just our own little group?
Honestly there's so little presented here that it's an idea I don't see working on its own, it needs a lot more to make any of this even slightly add up.
