The great beyond. Or, if you'd prefer, the known galaxy.
"Magnus" is the term coined by the Valen for all charted space. Although few have adopted such a saying, "what in Magnus" would be an appropriate alternative for space-faring folks to the terrestrial "what in the world" remark.
Spacefaring craft use Magnus charts to determine their location based on electromagnetic signatures of nearby stars. Their strength, relative position, and scale relative to a ship's sensor array allow ships to quickly identify their orientation, position, speed, and more. These charts are frequently updated by scientific craft to include more of the galaxy. All conventional navigation equipment uses these techniques, removing all need for radio communication to determine one's location.
Magnus contains all major solar systems of all space-faring races, as well as numerous neighboring solar systems. The distances between these systems is immense, requiring FTL travel techniques for the distances to be manageable. Communication, however, is another story - see The Exogrid for more details on that.
The Valen contrived a solution to "faster-than-light travel" in their early days of plotting out the universe, investing far more time into perfecting this technology as trade became a possibility. They created the SENT (Slow-Entry Networked Transit) Network, a network of wormholes placed strategically throughout the universe. These are highly stable wormholes allowing transport of ships, personnel, goods, and just about anything through them. In this way, the SENT network acts like a highway system for the known galaxy.
The main drawback with the SENT Network is that ships must slow to a slow relative speed to the SENT - often less than 100m/s - to be able to pass through. This is primarily due to the particular technique used to stabilize these wormholes. All matter that attempts to pass through faster than this is effectively torn apart and vaporized. SENTs are not effective methods of fleeing dangerous situations due to this tradeoff.
There are no other known flaws with SENTs. The Valen (as well as other races) are continuously working on better technologies than the SENTs. However, there have not been any new technologies publicly proposed to replace them.
As the name implies, all uncharted space is referred to as "Outer Magnus." Charting expeditions have traditionally mapped the outer edges of charted space in wide sweeps, resulting in somewhat of a smooth, bean-like shaped chart. Outside of this zone hazards are not mapped, repeaters are far and few, and help is hard to come by.
As only purposely-designed charting craft can properly create new charting data, sticking to charted space can be instrumental in avoiding navigation equipment failures. These techniques rely on "best fit" algorithms - techniques that can fail when no data fits the charts properly. Inexperienced pilots with low quality navigation gear can easily become lost, and eventually stranded, in Outer Magnus if they rely solely on this algorithm based navigation.
Within charted space, there are records of wide variety of potential hazards within the galaxy. The most obvious of all hazards would be simply debris of all kinds - asteroid fields, dust belts, etc. There's also plenty of traveler-made hazards out there to look out for, such as abandoned ships, destroyed space stations, and jettisoned cargo.
However, there are some less frequently encountered hazards out there. For instance, some nebulas can become so charged that they become an electromagnetic storm of sorts, wreaking havoc on any nearby, poorly shielded equipment. Flying into one of these can easily knock a ships computer's offline, destroy engines, disrupt life support systems, or even kill on-board crew.
As one heads toward the edges of charted space and enters Outer Magnus, they may encounter what are referred to as "dark zones". Scattered dust and gas particles of a negligible density in space in these zones can completely blotch out nearby star systems and other landmarks, making navigation almost impossible. It's possible to navigate out of these zones on radio only, but this is not always possible.
As well as natural hazards, there's plenty of folks equip with ships and crews with malicious intent out there. Magnus is vast, and space itself is larger. Nowhere is safe alone.