Their true origins are unknown, but that's never a satisfying answer, is it?
Formally, they are Alien Species 5-C. Informally, we call them the Idaltu. We owe them just about everything, or at least everything that we can't credit to the Oolians.
The main thing they gave us: fold vectors. How were they built? Nobody knows. For a while, we thought they were natural phenomena. After comparing notes with other species, though, it became pretty obvious these things were anything but natural. They also didn't line up with the rest of our physics. They had no electromagnetic emissions, no detectable properties at all. You had to know where one was in order to activate it. Now, with the right equipment you could tickle one just enough to be sure it was there. A lot of mapping was done that way, but plenty of others had beaten us to it in most cases.
Right, so the vectors themselves: zero-dimensional hyperspheres, or more easily described as entangled pairs of points in spacetime. Every single fold vector is linked to another one somewhere else. It's entirely a point-to-point network. I wasn't lying when I said they have no gravitational properties or anything like that, but they do respond to the outside universe in some interesting ways. For one thing, they are gravitationally bound to nearby celestial objects. They're usually at or near the edge of outermost planetary orbits in a solar system. The points themselves orbit, too, and every ship's computer can do the calculations to figure out where a known FV is supposed to be right now.
Knowing where an FV is means nothing if you don't know how to use it, though. That's the part the Oolians had to teach us. The lore goes that the Oolians stole the knowledge from the Koraxians, who stole it from the Idaltu. It's possible there were some other species in between, to be honest. Koraxians don't like to talk about their history much unless it's to say something very, very fascist. But I'm not talking about them right now, I'm talking about the Idaltu. Actually, I'm talking about fold vectors, so let me finish up that point and then move on. That "point" wasn't supposed to be a pun, but I guess it is now.
What you might call a "fold vector drive" isn't a conventional engine at all. It's a complex series of metal plates created using a highly sophisticated alloying process. In a specific arrangement and charged in a particular fashion, they generate a Casimir effect as you would expect. The plates start drawing toward each other, but they're held in a scaffold that doesn't allow for any actual movement. Instead, this force is tuned rapidly, almost like turning a key after putting it inside a lock. You tune it just right while in proximity of the FV point and it will activate. It will move a perfectly spherical volume from the current FV to its entangled cousin instantaneously no matter how far away it is. No, our ships aren't spherical--that would be silly. We just modulate the field such that an entire ship is encapsulated within the transportable sphere. This does mean that if anything else is in the volume with you, it will come along for the ride. Sometimes convoys use just that trick: they have escorts which have very powerful FV drives and take all of them at once, leaving extra room in the cargo ships for, you know, cargo.
Of course, if you are in the middle of a battle, you could also take some bad guys with you. That isn't very fun. I've done it a few times, myself. Can you slice a ship in half if you trigger an FV while only part of the ship is inside the volume? You bet! Done that, too. Some species have designated this a war crime. I have yet to be charged.
How many of these FVs did the Idaltu create? Thousands across the Milky Way, as far as we know, possibly tens of thousands. By no means have they all been mapped. Reiterating an earlier point, the Oolians got reasonably good maps out of the Koraxians, who got them out of whoever. Some other species shared what they've discovered over the years, too. FV maps are solid bargaining chips in trade and diplomatic negotiations. We have no idea if the Idaltu felt that way about them, if they had neighbors who lacked such technology and were envious, or what.
So, most of what we know about them comes to us by way of the Oolians, who speak of them with immense reverence. The word they use for the Idaltu, impossible for human tongues, means something like "builders of ancient roads," which is simultaneously poetic and obvious. I suppose that's a good way to describe the Oolians themselves. They're fairly well-meaning in general.
The rest of what we know about the Idaltu is pretty scattered. The greatest clue to their existence is a planet found in the Large Magellanic Cloud. First of all, the fold vector to reach it is in the middle of nowhere. It's in the mostly empty space between the Sagittarius Arm and the Perseus Arm, and there's no good reason anybody would want an FV there except to make sure nobody else would stumble across it. The Oolians said they bought the info from a couple of guys who knew way more than they should've but weren't willing to share much else. They neglected to mention the price paid and seemed rather embarrassed about it. In any case, the FV was legit and they took a ship through it. It brought them to an otherwise unremarkable spot in the middle of the LMC that just happened to be a million or so kilometers from a rogue planet. Such a find wouldn't be that interesting in and of itself except that the FV was calculated to be bound to the planet, so the former was definitely put there on purpose. Secondly, the planet was a perfect sphere, rather than slightly squashed into an oblate spheroid. No planet known to anyone has ever had such a nature.
Third, despite being a rogue planet, it's self-illuminating. The energy source is believed to be under the surface. How far, nobody knows. Some assume it draws energy from the spinning of its core, which is a reasonable enough guess. If true, it could stay lit up like that for millions of years.
It's not lit in just any way, though. In particular, the surface is a smooth gray, almost gunmetal, except it is inscribed with what can only be described as writing. It begins at one of the poles and basically spirals out in a tight coil all the way around the planet, around and around, until coming down to the opposite pole and terminating there. Every linguist in the galaxy has taken at least a few cracks at trying to translate it. Each of the assumed written characters covers several square kilometers. The shapes are complex enough that we don't think they are letters representing phonemes, but more likely abstracted pictograms, like a form of hieroglyphics evolved into a vastly different form over time. Out of about 20 million glowing characters etched into the planet's surface, there are roughly 5000 distinct forms. Suffice it to say, anyone studying this thing has their work cut out for them. The length of the document--if we can call it that, though it's definitely the least portable document ever written--is tantamount to a couple hundred novels. Given that size, our best minds also guess it is a repository of knowledge or maybe a detailed history of their people. Others think it's a technical manual, given the sheer engineering skill required to create a planet like this and draw a massive tome on it. It wouldn't surprise anyone to hear that once humans learned of the Idaltu Tomeworld, some got really religious about it. There's a cult of seventy thousand people or so who devote their lives to finding religious significance in the writings. So far they have alternately concluded it is a divine guide for how all beings should live, a prophecy of the end times, or an extremely long genealogical record.
There are some other weird traits of the planet, too, but they're probably less interesting than it having writing all over it. It has a strong magnetic field that protects it from just about anything that might fly too close, and in fact is so intense that few ships can approach without their instruments and navigational equipment going haywire. Every once in a while some errant scholar gets a little too close, loses power, and has to be grappled out by rescue ship. It's almost a rite of passage for PhD candidates nowadays.
You could have a laugh at how something like this is almost a tourist attraction. It's kind of like the Eiffel Tower, though: more impressive in pictures and the imagination than in person.
Suffice it to say, nobody has ever managed to land on it. Probes fired at the surface vaporize before they can land. The planet has no atmosphere, which is a little strange considering the powerful magnetosphere, but it's possible it just never had one or was intentionally stripped of it. We're not convinced it's a natural planet to begin with. It could have been built wholly by the Idaltu. It could be hollow, a Dyson Sphere around a red dwarf, but I'm told the physics of that are questionable. The interior is simply unknown to us.
The last thing I'll say about the Idaltu, for now, is that nobody has any evidence for what they looked like. The running joke, however, is that they must have been enormous. There are artist renderings that go around showing these enormous, goofy-looking aliens, drilling into the surface of a planet with a "pen" that's a gigantic metal rod. For scale, Jupiter would be about the size of a basketball to such a being. Did they bat planets around for fun? I like to think so.
Some of the weirder conjectures are that the Idaltu are the fold vectors, that their very essence in some way seeped into the fabric of the universe and tore tiny holes in the walls of reality, maybe in places that were interesting or notable to them. Maybe they're still watching us. Maybe every time we use a fold vector, they become slightly more aware of our presence. Would they find us interesting or disregard us as little more than germs? We just hope they wouldn't be hostile, if they really are out there somewhere. If they're as big the satirical drawings, they could just literally sit on any planet that irritated them and it would be game over for billions of beings. Funny, but not really.
Yeah, I personally hope they're all dead. There's enough other weird, dangerous shit in the universe to worry about.