It is called the Outrim Void. The Borderland. Wild Space.

If the Spinward Marches is an isolated outpost to the rest of the Imperium, then the Trojan Reach is where the wilderness begins. The sector is an expanse of uncharted worlds and petty, backward barbarian kingdoms, a chaotic, treacherous zone of space teetering on the maw of the Great Rift. Somewhere, out there beyond the little one-world kingdoms and the failed Second Imperium colonies, are the ferocious and fearsome Aslan, unrestrained by the Peace of Fthair.