Exploration in the game now, is not how it was initially planned and promoted.
Regarding Thargoids and Guardians etc, those were all narrative based inserts, and as others have confirmed, follow rules, but not all of them have been found.
Initially exploration was to be economically and politically linked, and there was a really cool mechanism involved, but for various reasons FD binned it…
What we actually got was incredibly vanilla.
In short from day one, nothing existed outside the bubble except procedural systems, most mysteries existed in the bubble…until FD turn stuff on.
Exploration flag planting was the norm from day one and a lot of players did a great deal of work very quickly; however they had limited resources back then so in actuality, they did actually miss things. You can still go to ‘discovered’ systems and find things others have missed.
Regardless exploration is what you make of it. It’s basically not an active mechanic…
If you feel the need for purpose, look at some of the exploration groups and get involved in some of the research being undertaken….
There’s actually stuff out there that Cmdrs aren’t aware of…and FD aren’t talking about…