Wilderness Space is the name officially used by the Imperium of Man to refer to all those areas of the Milky Way Galaxy and beyond that have not yet been explored or charted by the agents of the Emperor of Mankind.
There are many reasons why so much of the galaxy remains unexplored even in the late 41st Millennium. First, the galaxy is still a truly massive region of space-time on the Human scale, and even after almost forty standard millennia of space travel Mankind has not yet been able to pierce every dark corner and hidden stellar cranny.
Like a terrestrial ocean, the Warp has currents, eddies, and storms, all of which can block access to vast regions of the galaxy. This is particularly true in times when the power of the Ruinous Powers waxes, as the resulting growth in Warp Storms and Warp rifts like the Maelstrom, the ancient Ruinstorm and the current Great Rift can cordon off entire segmentums from each other.
Other, outlying regions of the galaxy, such as the Halo Stars, are simply judged to possess neither natural resources or environments suitable for Human colonisation. Thus, neither Explorators nor Rogue Traders bother to make the journey, seeing no profit in the venture.
Finally, certain regions of the galaxy, particularly in the Eastern Fringe, are so far from Terra, that they fall beyond easy reach of the Astronomican. Unable to sense the Emperor's guiding psychic beacon, many Navigators refuse to enter these regions, fearful of being lost in the Warp for an eternity.
- Codex Imperialis (2nd Edition), "The Galaxy"