Downward Horizon
The Downward Sector of the Milky Way galaxy is the least explored sector of the M 0 Galaxy and the imaginary line between Downward and Coreward is often called the Downward Horizon. The so called Downward Horizon is marked as such in Peterson and Ty'tyhc stellar maps and corrected every year to reflect the collection of knowledge.
Basically it means that there is unknown and unexplored space beyond the Downward Horizon.