Currently reading, In My Copious Free Time: Anathem.
I'm up to page 407, rather less than halfway through. Time to start guessing what's ahead!
Here's a guess: the Cousins are Russian-analogs (Russian on account of the writing on their ship). During the Terrible Events, some of them, foreseeing their annihilation, bunged themselves into an immense icosahedral Orion-drive spaceship and launched themselves into the void, perhaps on a lengthy trajectory which, all these centuries later, has brought them back into Ardre's neighborhood.
It could be a generation ship, or perhaps there's a Thousander longevity secret to which they're privy. Maybe both.
Anyway, it's a sort of flying near-quadramillennial math, about to open its gates to see what's happened in the world.
That seems to fit all the known facts up to this point. Reading time permitting, I'll eventually learn what the author thinks that stupendous stone spaceship is.