Finding Raxxla, should it exist, will be like assembling a 10 thousand piece blank jigsaw puzzle. All one can go off of in that case is the shape of each individual piece or, perhaps, the smudges and blemishes left by the creator of the puzzle. MB's "no clues" line makes that much clear. So what are the smudges? What are the shapes? I think it would pay to think about this in a more "empathetic" way, if that makes sense - put ourselves in the shoes of whatever caused or created Raxxla. As I see it there are two avenues to take here.
1 - In-game empathy. Whichever entity is responsible for Raxxla, be it an active force or something more passive, must have operated under some set of rules. If it is a passive force (Raxxla as a creation of natural origins) then we must consider physical laws and phenomena. If it is an active force, then we must consider the possible motivations for its creation and the logical steps a creator may take in doing so, such as where they might place it or what they might do to hide it if they deem it necessary. Virtually every decision is based off of some rational calculation.
2 - Out-game empathy. Think like a dev! If you were tasked with storyboarding the Raxxla mystery, how would it play out? Consider how FDev has handled the story of the Thargoids, or the Guardians or the Formidine Rift.
In addition to this, we have the precious few mentions in the lore which do little to direct a search, but perhaps give us a sense of what Raxxla is. Together, we have at least some process by which to search (the empathetic approach in looking for the location, coupled with the lore mentions to give us an idea of what we might find at the correct location).
As for the lore, I think it's clear we're looking for some type of planet - but I wouldn't rule out other bodies, specifically black holes. There is certainly a lot of sci-fi surrounding the possibility of black holes being gateways, and wormholes also show up often in a similar context. Even real science admits in some theories the possibility of inter-universal travel under very specific conditions involving black holes. White holes are another option - matter from other universes could enter ours from there. While not allowing direct travel, one could effectively gather bounties and treasures as mentioned in the lore from a white hole (at least in science fiction). So keep your eyes open for other phenomena as well.
Moving on to the empathetic side of things - place yourself in the shoes of Raxxla's constructors (let's assume they are active forces, rather than passive forces such as physical phenomena). Why would you want to build Raxxla? To gain riches, power, or some other boon, presumably. Or perhaps to hide... Whatever the case, it logically follows that one would likely seek to hide such a construct. These tidbits so far are nothing new in the search for Raxxla. If either was not true, then Raxxla either would not have been hidden well and thus been found, or it would not have been terribly important to begin with and thus not occupy so much myth and legend to begin with. But it's what comes next where things get muddled. Simply put: Where would they have hidden it? There are a few overarching options that stand out:
1 - Hide it in plain sight. Always an attractive option, it turns those searching in the traditional grand sense (long range treks to the far corners of the galaxy) on their heads. It is a trickster's choice, exactly the kind of move we might expect from the shady beings behind Raxxla. But in the context of the devs, not much has ever been in "plain sight" when it comes to searching out mysteries. The closest thing would perhaps be the Zurara - after years of searching the Rift, it turned up exactly where the search began - along the RR line.
2 - Hide it in a nebula. Drew Wagar mentioned during the Rift mystery that Nebulae were great hiding spots, or perhaps staging points. And in-game, anyone can see how a nebula could obstruct vision, eventually taking up whole swathes of one's view and blotting out entire patches of stars. It would make sense to put Raxxla in or near one of them. Humans seem to like setting up shop there, as do the Guardians and Thargoids. Raxxla's creators may have had the same thought. And devs may be attracted to placing Raxxla in or near a nebula for its story-telling potential. But then again, it's an obvious choice, which works against the likelihood of it. Furthermore, if Raxxla was in a nebula, the past player-led searches of them may well have turned it up already.
3 - Hide it in the far-off void. Make Raxxla as hard to reach as possible - hide it behind jumponium-necessitating gaps in the stars, perhaps even beyond the edge of the galaxy. The appeal is obvious here. But it would likely necessitate some clues when it comes to finding it, and we know that there will be no clues. Either this discounts this option, or makes it all the more likely - perhaps the devs' point of view is that it should be essentially un-findable without a massive stroke of luck fitting for the galactic scale of the mystery.
Of the three, it is hard to choose, but if I was forced to bet I would pick 3, with 2 and 1 being pretty much a tie. Regardless, all options must be examined. I suppose this is a somewhat unsatisfying conclusion to a wall of text, even to myself. I was hoping a thorough walkthrough of the information would help me narrow stuff down, but it has only done so slightly. I guess if it counts for anything, if I had a gun to my head I'd point to Bovomit, but until that's unlocked (or we find a way to bypass its permit lock) it'll have to wait. And it's only a gut feeling anyway.