Nexus

A nexus is a piece of infrastructure used to travel between different star systems. Such travel is not constrained by the speed of light.

The Prime Nexus

The Prime Nexus is the originator of The Empyrean. Very little is known about its origins, despite extensive study. The network of the Empyrean feeds through it, in and out, recycling constantly. This is made possible by nexite, an engineered material. It is 'mined' across the galaxy, but the mining sites actually construct it as part of the extraction process and this isn't common knowledge, so most people think it's just something you can dig out of the ground.

Construction

Each nexus consists of at least two tapered tubes. These are made of nexite, but sometimes plated with another material on their outer surface. Many have had Weapons emplacements or refuelling stations attached as well, given their importance in galactic travel.

Function

Each nexus uses the Empyrean to accelerate ships through its tubes, punching them out across the galaxy as far as its range will carry them. This happens along what The Hegemony call Varna tunnels, which are calculated by the nexus prior to a journey to avoid known celestial bodies. Anything smaller is punched out of the way en-route by the Empyrean energy propelling the craft, although such impacts can make a journey take longer or even end prematurely (though this requires precise conditions and is very rare).
There are two theories as to how ships avoid crashing into other ships during inter-nexus travel:

1. Nexuses propel vessels through a higher dimension of space – this makes sense given they disappear when crossing the centre point of the departure nexus and reappear when crossing the centre point of the destination one
2. Nexuses are capable of maintaining an accurate set of galactic routes for ships to avoid. Since the only 'reasonable' intersection points are at nexuses and the lanes around them are automated/regulated, the nexus itself can make sure nothing is in the way

Expanding from the Prime Nexus

Since a nexus can only get you so far and once you've used it, you don't have a fast way back, you need to have a nexus waiting for you on the other side. What records survive indicate that the first nexus (after the Prime Nexus) was created by taking a chunk of the Prime Nexus and propelling it through to a system targeted for colonisation. The Prime Nexus is built to repair itself, so that wasn't a major problem. All they had to do was make sure they synced the AI between the two of them before leaving. Newer nexuses still need to have a copy of the base AI taken from another nexus – preferably the Prime Nexus, but other nexuses suffice.
The Prime Nexus can be fed with nexite and other raw materials to construct another nexus, but as humans spread across the galaxy this become a problem – not least because The Protectorate had stolen it. The Hegemony had a lot of nexite, but no Prime Nexus. The Protectorate had the Prime Nexus, but a limited supply of nexite. Eventually, the Hegemony figured out a way to use an existing nexus (not the Prime Nexus) to manufacture other nexuses. It takes a lot longer than it would do normally and is limited by the size of the original, but it works. The Hegemony have a nexus dedicated to constructing other nexuses. When not in use for this, they either move it between systems they want to temporarily exploit for Resources or explore or they loan it out to other factions (not the Protectorate). The Laresic Merchant Republic only got started up properly because someone rich decided to hire it out for a while during a gap in colonisation efforts.
As such, Hegemony nexuses have a short range, but there are far more of them and the mobile nexus gives them the advantage of flexibility. The Protectorate nexuses are considerably more powerful, but they don't have the resources to make as many. The Prime Nexus could reasonably punch as far into Hegemony territory as Empyros from Hesperex (not advisable, unless you have a scrambler to pretend you came from somewhere else, and such tactics only work on small numbers of craft that aren't subject to much scrutiny). An ordinary Protectorate nexus could possibly get from Chivla to Varn in one jump. A Hegemony nexus would probably only make it from Varn to Rembra.
In any case, since these other nexuses form part of the same network and can direct the flow, it is theoretically possible to 'tug' the Prime Nexus out of position. At one stage it was possible to move the Prime Nexus under its own power, but no one alive knows that was ever possible, let alone how to do it.