Milky Way

The Milky Way galaxy has yet to be fully explored. Whilst the overall appearance and structure of the Milky Way has been known for centuries, hyperlanes to most of the galaxy are yet to be discovered. For example, the Perseus Arm so far has a single hyperlane leading to it in the Perseus Ultima system, whilst the area of of the Sagittarius arm known as the Expanse has no access hyperlanes. 
The known galaxy refers to areas of the galaxy which are accessible to the galactic nations. Explorers have over time found that vertical hyperlanes connected virtually all systems above and under the galactic plane, meaning that most of the exploration was to be done on said plane.
The known galaxy spans from the edge of Terran Territories to the end of the Mirdesh Confederacy, or about 60kly in length and 10kly in width.