Chicken and Egg
The material is theoretically understood by everyone, and its creation involves something akin to a fusion reactor, which can be done efficiently if you already have enough of the material, but there are such small amounts present in stolen items that this is not practical. (Perhaps it needs to be in the form of giant single crystals/pieces/rings which cannot be reassembled from smaller pieces without a reactor.)
This option means that an intelligence operative cannot just observe the process; a large quantity would need to be smuggled out.
Obviously the faction had to produce the initial stuff somewhere. Perhaps they dug it up in ancient ruins, or it's from a meteor. Or, a secret military research discovered a method of doing it using extremely expensive conventional technology. But as long as this "conventional technology method" remains a secret, nobody else can replicate it. They should probably destroy that original facility.
Secret process
In theoretical physics there are many particles that we have never been able to create, and some that we have never observed in nature but are believed to exist. If one faction discovers how to make something, they can then exploit this knowledge. 159
Especially if we add in the "chicken and egg" thing.
Let's say for argument's sake that there is a recent development in physics, which may or may not be known to other factions, but either way none of them have been able to exploit it yet. The winning faction has built a huge facility with half a million carefully-arranged parts and the "blueprints" are stored in bits and pieces in different places.
There could be many ideas here but here is one of them. Let's call it a "black disc", a tiny (and reasonably safe) planar black hole held together on either side by a carefully engineered dense perfect lattice of particles that produce the right repulsive forces to stop the disc from collapsing to a point. How the heck do you build that lattice?
Perhaps these particles are beyond its event horizon; or perhaps they are just subject to such high gravity; but any attempt to scrape some off fails. So the lattice cannot be analysed.
Even if the principle is understood, designing a manufacturing process requires particular flashes of insight. It isn't simply something you throw dollars at. Either the initial scientist(s) found the trick out of sheer luck, or the zeitgeist was just right and nobody else can replicate it. 284