It depends on your point of view
Like many of the truths we cling to, this depends on one’s point of view. There are varied views of the Force, with Jedi and Sith holding opposing views on what the Force really means.
It is certainly the opinion of at least some among the Sith, even in canon, that the dark side is the purer portion of the Force, its true nature. Vader believes that the Force "mandates order," and that the Jedi misunderstand its nature:
Hate was the font of true strength. Suffering was not the result of
the rule of the strong over the weak, order was. By its very
existence, the Force mandated the rule of the strong over the weak;
the Force mandated order. The Jedi had never seen that, and so they’d
misunderstood the Force and been destroyed. But Vader’s Master saw it.
Vader saw it. And so they were strong. And so they ruled.
Lords of the Sith
He even believes that the Force requires that order be imposed through violence, thus ascribing a distinctly dark side element to the natural state of the Force:
It was his duty to rule them all. He saw that now. It was the manifest
will of the Force. Existence without proper rule was chaos, disorder,
suboptimal. The Force—invisible but ubiquitous—bent toward order and
was the tool through which order could and must be imposed, but not
through harmony, not through peaceful coexistence. That had been the
approach of the Jedi, a foolish, failed approach that only fomented
more disorder. Vader and his Master imposed order the only way it
could be imposed, the way the Force required that it be imposed,
through conquest, by forcing the disorder to submit to the order, by
bending the weak to the will of the strong.
Lords of the Sith
Of course, Vader doesn’t deny that there exist people who don’t use the dark side—he does, after all, occasionally use that term himself: he merely believes that they misunderstand what the Force really is, and thus limit themselves.
The Jedi undoubtedly take the opposite point of view, that the dark side is a perversion of the true nature of the Force. They generally speak of "The Force," not "the light side" (much as Vader does, interestingly), and refer to "falling to the dark side," but rarely "adhering to the light" or some such.
Even some Sith Lords seem to agree. Palpatine seems to take a different point of view from Vader. He seems to view the Force somewhat as the Jedi do, believing that it might "strike back" against his aims, implicitly equating the Force with the light side.
Darth Plagueis had once remarked that “the Force can strike back.” The
death of a star didn’t necessarily curtail its light, and indeed
Sidious could see evidence of that sometimes even in Vader—the barest
flicker of persistent light. Attacks like the one directed against
Tarkin’s moon base and discoveries like the one on Murkhana were
distractions to his ultimate goal of making certain that the Force
could not strike back, and that whatever faint light of hope remained
could be snuffed out for good.
Undoubtedly he believes that the dark side is stronger (or at least leads to greater personal power), but he doesn’t seem to be talking about the entirety of the Force as if it were the dark side (as Vader does).
Like much about the Force, though, we’re not really told who is right. There is not one unified view of the Force among even the Jedi, or for that matter the Sith. Everyone agrees that there are certain powers that can be acquired only through the dark side, but they don’t agree on what that means. Does it mean embracing a discrete aspect of the Force? Recognizing the “true nature of the Force,” as Vader said to Luke? Using an amoral instrument in accordance with one’s darker impulses? Each of these ideas has its adherents, both in Legends and to some extent in canon. And similarly with the light side.
Some people, such as Vader, seem to believe that the dark side is the real nature of the Force, and the view of the Jedi is mere delusion. Some view the Force in dualistic terms, but view the dark side as more powerful. Some view the Force in dualistic terms, but view the light side as more powerful. Some view the Force as the light side, and the dark side as a distortion. And so on and so forth. It’s not really certain who’s right.