In Traveller the Imperium charted space is a small portion of the galaxy. In the T5 book, the Galaxy map (page 417) has Abyssals and Denizens west of Core, Dushis Kurisi North of Core, Essaray east of Core, The Barren Arm south east of Core and Core Sophonts in the Core.
What else is out there and who are all those listed but unexplained sections?
Far Future released this short PDF called The Galaxiad that provides sketchy information on names mentioned in the Traveller Galaxy. Marc Miller explains more in this Kickstarter Post.
Integral with the creation of Traveller5 has been the construction of the Galaxiad… a multi season epic adventure exploring the Galaxy, encountering new intelligences (and even that word is misleading), history, background, dangers, and threats, all with your characters at the center.
There is little written about anything other than the roughly dozen fully detailed sectors, and the UWP generated 40 sectors or so. And much of that material isn't fully canonical.
The material in T5 was a surprise to most of the playtesters, including me. And for what its worth, aside from Marc Miller, Rob Eaglestone and Don McKinney, almost no one has any clue, and Rob and Don aren't saying.
The CT AM Zhodani notes that the Zhodani have penetrated more than 4000 parsecs coreward... (It's about 8000 parsecs to the core from Earth. 200 Sectors.)
Travellermap.com shows the Zhodani Core Route, along with a mixture of dot-map and generated map systems the whole way.
From AM4: Zhodani, p. 10.
Vlanchiets Qlom: Vlanchiets Qlom includes all Zhodani territory beyond Chtierabl sector- approximately thirty subsector and partial subsector-sized areas reaching 600 parsecs toward the core. It also includes a handful of bases established by core expeditions, some as far as halfway to the core. Portions of this province are as much as fifty years away from Zhdant at jump-4.
There is mention of a Solomani Rimward exploration as well, but its in Solomani & Aslan, by DGP, and may no longer be canon.
The stuff that doesn't touch the 3I hasn't been defined other than what's in the T5 Rulebook, excepting, as noted, the Core Expeditions reach.
Note that some fan initiatives have generated some data on distant locations; none of which is canon, tho' some is informed by canon strongly.
(1) Who else is out there?
It's a big galaxy. One of the dangers of T5's brief little overview there is that it might make the galaxy seem small and crowded. I suggest that five broad chunks of the galaxy labelled with new creatures/places to either visit or avoid, does not a full galaxy make. Just because the Abyssals are present across a quarter of the galaxy doesn't mean they have a galactic empire with borders, for example.
(2) Who are all those listed but unexplained sections?
They are prominent sophont groups who exist across astonishingly large areas of space. All five have parts to play in the Galaxiad. None of them are clearly good or evil; each have challenges of their own, and each can be particularly dangerous and helpful, depending on the circumstances... and perhaps depending on how First Contact goes with each of them.
And by the way, it may be that your players therefore operate in a different situation than other players: one where, for example, the Essaray are allies, versus one where the Essaray are standoffish... or one where they are eager to wipe out Charted Space.
The Barren Arm is the exception: it's a zone that is completely devoid of any sort of life at all. In Traveller parlance, it's a big enigma. Well, it's a big enigma to me, at least.
