You said you have FTL drives, but you didn't specify how they work (only said warp drive). They could use a special fuel to work, and since FTL is the most important thing in any galactic-size operation, this fuel would be the most expensive thing in the entire universe.
Just imagine being able to control the production of this special fuel, you could control entire empires with this unique resource. Everyone will be under your power, even more, you create and destroy with it (without FTL's fuel you can isolate an entire sector of the galaxy during thousands of years).
You could simply use unobtanium to feed your hyperdrives. Or if you like hard science, exotic matter:
Exotic mass has been considered a colloquial term for matters such as dark matter, negative mass, or complex mass
For being exactly, you could use a certain kind of exotic mass: negative mass:
Negative mass would possess some strange properties, such as accelerating in the direction opposite of applied force. Despite being inconsistent with the expected behavior of "normal" matter, negative mass is mathematically consistent and introduces no violation of conservation of momentum or energy. It is used in certain speculative theories, such as on the construction of artificial wormholes and the Alcubierre drive. The closest known real representative of such exotic matter is the region of pseudo-negative-pressure density produced by the Casimir effect.
That allows you to use both Alcubierre drives (warp drives) or traversable wormholes.
Allow me to perform a brief introduction to both FTL drives:
You are already using it so I think you already know something about it. Anyway, I'll write about it.
I've already explained it in this answer, but I'll add about the exotic matter:
The sci-fi warp drive or the currently theorized Alcubierre drive use the principle of space-time distortion (primarily based on gravitational distortions).
Rather than exceeding the speed of light within a local reference frame, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel. Objects cannot accelerate to the speed of light within normal spacetime; instead, the Alcubierre drive shifts space around an object so that the object would arrive at its destination faster than light would in normal space without breaking any physical laws.
Basically, you contract (bench downwards in 2D) space in front of you (so you could travel a "smaller" distance towards your destination) while you expand (bench upwards in 2D) the space behind you (enlarging the distance towards your start point). Contract space is quite "easy", you just need a lot of mass or a huge amount of energy (due mass-energy equivalence). Instead, expanding space require negative mass, and thus exotic matter.
As a minor failure the Alcubierre drive has some limitations:
- Mass-energy requiriment: A lot of negative mass, thought new speculations think it isn't so much, and so it's possible. Anyway, you must shrink it into a wall really thin, something a few orders of magnitude bigger than Planck constant, that is, really small.
- Placement of matter: Some theories require pre-made (subluminical made) high lines, routes or railroads to use the drive. Otherwise, you will need tachyonic matter, which scientists aren't very optimist about its existence.
- Survivability inside the bubble: During the FTL travel, since you are moving faster than light and in a space warped sphere, your ship's sensors get blinded, so you can't see if you are going to crash with something. Also, the hawking raditaion produced by the bubble (which I don't know how it is produced) may kill, smelt or "cook" for being exactly everything inside it.
- Damaging effect on destination: When the ship decelerates from FTL, the particles (like hydrogen) that the bubble gathered during the travel would be released with so much energy that they will blueshift into deadly radiation, effectively annihilating any life in front of the ship. Better not point towards Earth.
- Casuality violation: Like any FTL drive, the casuality could be violated due time travels.
It's your work see how to fix, avoid or just ignore the side-effects of the FTL travel.
If you would like to add portals, you could use wormholes.
You could watch this fancy and animated Kurzgesagt – In a Nutshell video about wormholes. Basically, to make a wormhole you need a massive amount of mass, enough to bench spacetime itself and tear it making "holes" on it. In order to do that, you need to compress a lot of mass into a single point.
After breaking the spacetime on that place a wormhole between to point of space will appear. That was the easiest part.
Now the problem is maintaining it. Due to the gravitation collapse of matter, the "tunnel" between that two points of space will suddenly close, at the same time both "mouths" of the wormhole turn into black holes. That is the difficult part, in order to move through a black hole, we need to surpass four things:
- Spaghettification: or noodle effect, the gravity of the wormhole/black hole increases so quickly as you came close that the gravity between two different parts of your body/ship isn't the same, and so you tear apart, turning into "spaghetti".
- Time dilatation: as you came close to the centre or singularity, the gravity increases and so time dilate, effectively taking infinite time to move through a wormhole.
- Event of horizon: the gravity is so strong, that after cross this line or horizon, you are no longer able to turn back. So, once you enter into a wormhole, you can no longer escape from any of both sides.
- Singularity: the gravity on this point (note that I said point and not a sphere) is infinite, the "tunnel" collapse, as time does and everything we know.
Luckily, all those things can be fixed with the same thing... you guess it! Exotic matter! If exotic matter (negative matter) works as scientists want to believe it will produce anti-gravity.
Mass object bench space "downwards" (on a 2D sheet) creating gravity and pulling objects near. Negative mass objects bench space "upwards" creating anti-gravity and pushing objects away. With enough amount of exotic matter and luck, we could "inject" it to a wormhole avoiding it to collapse due to gravity into a black hole, and thus avoiding any singularity, even of horizon, infinite time dilatation or, if the wormhole is enough big, even Spaghettification.
By the way, if you use any other type of FTL drive, like a jump drive, hyperdrive or hyperline, you could just use unobtanium.
PD: I am not physic, I might be forgetting/misleading/misunderstanding something.