@Mooing: bear in mind that the official game data has a significant "head-start" on EDSM data. (The game didn't launch with anything logged to journals, after all.) But how did it fare between the last two times Frontier released official data? Turns out that in the past 1.79 years, EDSM's activity of new systems covered 45.66% of the total. Let me quote an earlier post of mine for this:
Now, assuming that that figure is for [December 16], that means that since 2018 March 2, when FD tweeted that Commanders have discovered 112,863,791 systems, which was 0.028% of the galaxy, players have discovered +0.014% of the galaxy in the 654 days since. That would be about 169.3 million systems (169,295,687, but since we only got a percentage this time, there would be rounding errors), which is +56.4 million systems, or 86,300 new systems per day. How does EDSM stack up against that?
EDSM had 20,853,341 systems added on 2018-03-02, and today on 2019-12-16 it has 46,624,091, or 0.011652% of the galaxy. (Meaning that despite the long "head start", the whole database still covers 27.74% of the total.) So that's +25.8 million new systems added, or 39,400 new systems per day.
To sum it up: during the last 654 days (1.79 years), with regards to new systems, EDSM's activity covered an impressive 45.66% of the total.
Given the rule of the thumb that Darkfyre99 also quoted, that really is quite a lot. Exploration as an activity in Elite is something well worth analysing too, and we appear to have some evidence (like
DW2 statistics,
forum survey of veteran explorers) to suggest that the Commanders who are more committed to exploration share their finds to EDSM more than not.
Put another way: there is a
strong demand for some exploration features that don't exist in-game.
I think Frontier realised this themselves, or at least part of it, which was why they said back before Chapter Four that they'll be moving towards community exploration. Of course, all this meant was the Codex, which is... well. It took a step in the right direction, then fell flat on its face. We're waiting for it to get back on its feet for over a year now.
Anyway, back to the original point. From my observations (so this is only anecdotal "evidence"), when you go farther off from the beaten paths, the more likely it is that a system which wasn't uploaded to EDSM wasn't visited at all. For example, a month or so ago I surveyed the NGC 1931 nebula. A real one, not procedurally generated, and not even very far from the bubble, some 7,500 ly? (In other words, only 100 jumps in a 75 ly Anaconda, if you could fly in a straight line.) One of the lesser-known ones though, I guess.
Now, when I was looking around the Seagull nebula, which is only half the distance out
and has an asteroid base with a shipyard, about half of the EDSM-unvisited systems there were actually visited. Out at NGC 1931, less than a tenth of the non-EDSM systems were already visited. Many of them were discovered by one Commander, who turned out to be on Inara, just on Xbox and didn't upload these systems.
By the way, despite the popular myth of "all the nebulae are picked clean", I found virgin A/F/G systems out there, so mass code D even. Got a nice system with an ELW and an AW-WW binary pair, too.
Now, to my main point with this: suppose that you wanted to do a statistical analysis of the NGC 1931 nebula subsector. Given these tendencies, and taking a look at the visited stars there (see
VisitedStarsCache.space), would you have enough data already? I think yes. Then take a look at the Seagull nebula I mentioned earlier. (I spent some time there

) Would you? Definitely.
The systems missing from EDSM but discovered in-game are not evenly distributed. Areas of (higher) interest are more likely to have been uploaded to EDSM too. Now, depending on what you're analysing and what kind of conclusions you're looking to draw, I think that for many of them, we have enough data already.
On to the examples you mentioned. "Imagine if we had 3.5X more data, 3.5X more planets, more stars, more Earth-like Worlds, more NSP's, more GGG's " First, bear in mind that
we don't have official data on stars and planets from Frontier, so
we don't know if the same ratio holds for them as it does for systems. If we had 3.5 more Earth-like Worlds than we do now, well, I've looked at the data plenty, and there aren't many more conclusions to be drawn from it that could be done with 3.5 times as many as we have now. (Some suspicions on their distribution would be better confirmed or refuted, but the practical value of that would be little.) As for NSPs, thanks to the Codex, we know every
kind of NSP contents that have been found, and they're all on EDSM, because somebody will visit them. For GGGs, I wouldn't be surprised if all of the discovered ones were on EDSM already, but even if there are some lurking in regions where one has been found already (and thus, the Codex not listing the subsequent ones), it would be a good bet that they wouldn't make up thrice the amount that we already know of. Those are all already on EDSM too.
So we're left with 3.5 times more stars, since those are auto-scanned now. Yes, that could be very useful. At the current rate, I suppose a bit under half the discovered stars are uploaded to EDSM too. (Since the auto-scanner will add all of them from every visited system, we can use the systems ratio.)
What we already have is quite impressive already. If you want to improve how many people upload to EDSM, there is a thing you can do: tell people new to exploration about it, and help walk them through how sharing your finds there works. It really is quite easy.
The only way it would be easier if the game itself had an "upload to EDSM" button after you sell your data, really.