While I haven't read all the EU stuff by any means I've got a different take. "Balance" as it's used in the POTC (yeah thats right, initialisms) is super vague and never well defined- and that seems pretty much by design. I see bringing Balance to the Force as the removal of the Sith corruption to the neutrality of the Force. The Force is an aspect of nature that certain individuals over thousands of years learned to perceive an interact with. Then some nasty people figured out how to weaponize it. The Prophesy is nature returning itself to balance.
It's a Jedi legend about the Force returning itself to what is was before it was tainted by ancient dark Jedi who nudged the Force away from it's natural harmony. Anakin is the one who returns balance to the force by killing Palpatine and in so doing killing himself- No more Sith, no more imbalance and the Force returns to harmony and impartiality.
There is a defined history about all of this. In fact, there is so much written about "the prophesy" that we really do not have to guess really about any of it.
The story behind the Force is that it is a potential "life-force" that resides in all living beings. The Rakata race learned to harness this force and simply called it "The Force." They used the power of the Force to create a massive military armada. This fleet allowed them to expand, eventually conquering the entire galaxy and establishing The Infinite Empire. They enslaved all species they came in contact with.
This is how all the species in the galaxy came to know the Force.
Once the Infinite Empire fell (for various reasons), each species was left to study the Force on their own, and they developed different schools of thought over time as how to best utilize it.
Anyway.. that's the very abridged history of the Force according to the EU.
But the Force is actually based on Taoism and the ying and yang according to Lucas. Watching his Behind the Scenes commentary, you can see Lucas also incorporated imagery and concepts of Zoroastrianism.
Concisely, understanding the dark/light side duality is similar to understanding Taoism's yin and yang concepts and Zoroastrianism's "dark vs light" narrative.
However, contrary to Zoroastrianism but very much in congruence with Taoism; neither side of the force is actually "evil," and you hear Lucas say this himself. The dark side of the force is
not and does not represent evil because true evil doesn't really exist in Star Wars.
What matters are the choices you make and how you choose to use the power that you have. That's the entire point of Star Wars in general. Lucas is continually faced with choices.
In Episode IV, we see this throughout the film, deliberately:
1) do you fight and avenge your aunt and uncle or do you disappear into the desert;
2) do you help the Princess or do you find the nearest spaceport;
3) do you take up Han on his offer and leave with your reward, or do you forgo the reward and fight alongside the Rebel Alliance;
4) do you choose to embrace technology as you did earlier in the movie - or do you finally choose to "let go" and embrace the Force?
In Episode V and VI the choices become more grave and Luke is introduced to the dark side of the Force, but again, those movies are filled with Luke being faced with decision after decision requiring his affirmation to one choice or a diametrically opposed alternative.
Lastly, according to Lucas himself, the prophesy simply means all the Jedi and the Sith are gone. That would represent balance. The Jedi themselves are an imbalance; just as the Sith are an imbalance. This is alluded to when Yoda tells Mace Windu and Obi-Wan that he suspected they misunderstood and "misread" the prophesy. That perhaps "The One Who Would Bring Balance to the Force", does not mean the one who would kill
just the Sith...