Koronus Expanse
The Calixis sector is bordered to rimward (in this case that's the same as galactic north) by the Koronus Expanse. This vast region stretches from the outer edges of the galactic disk and into the galactic halo, where young vibrant stars are rare, and dark and terrible things altogether more common (here be monsters the charts say). No one knows exactly how big the Expanse is - it simply hasn't been explored sufficiently for anyone to say - but it is vastly larger than any Imperial sector, comprising billions of cubic light years and the God-Emperor knows how many stellar and planetary bodies. At the very least it covers the outer margins of the Cygnus arm, from around the position of Caliban (due north of the Eye of Terror) and all the way to Segmentum Pacificus.
The reason for this lack of exploration is the massive warp storms that still boil along the northern borders of Calixis sector - the aftershocks of the storms that savaged the Scarus Sector ages ago. Indeed, there is only one commonly known stable passage into The Expanse - the Maw - whose discovery came in late M40. At first exploration and exploitation was slow, but as the Calyx Expanse (now the Calixis Sector) became ever more civilized the adventuresome searched outward, beyond the Maw. In late M41 the regions closest to the Maw have been explored and worlds settled, but further afield there are opportunities awaiting the ruthless and the bold!