I like the shortcut fast-travel bubbly blue windows 
The arches are indestructable to all things (except the halos), and ancient humans build many cities along the arches (so it can be assumed that either they provided some atmosphere or the arches already have a atmosphere)
The Domain is a information network not located in normal space-time (presumabally slipspace?) that the forerunners use to archive all their history
The Flood revealed to the Librarian that the Domain was the long mythologized “Organon”, which is a precursor relic that had the power to control all other precursor tech
the Domain/Organon also happened to be a Precursor consciousness
and apparently, it survived the halo firings (as evidenced by the terminals in the games)
What exactly do these arches look like? I’m just sort of imaging something like a giant version of the gateway arch that extends into space, and maybe sets down in another dimension or something.
the ISO-didact (bornsteller) describes the Orbital Arches and Unbending Filaments on Charum Hakkor as “ivy growing on great trees”
they are also referred to as “star bridges” in the books
basically that giant arch thingy in St. Louis
That is what I meant by gateway arch.
could’ve been a inspiration
just like how the halos were from the “ring worlds” book
Just started reading Silentium a few days ago. It’s easily the best book of the Forerunner trilogy by far.
Plus we finally get to understand what Guilty Spark meant when he told Chief, “Last time you asked me, if it were my choice, would I do it?”
… only the makers of Halo would wait 12 years before explaining one of the most cryptic lines in the games…