Ron Moore compared the Dominion to China that implies to me the dominion controls a substantial portion of the Gamma Quadrant and probably has a tributary relationship with the rest of it. 19% charted? That seems rather strange given the intense exploration conducted in the past oh 200 years or so. Maybe charted means thoroughly catalogued every rock and flora.
There are blind spots substantial portions of the Beta Quadrant are unexplored, and there is no telling what you would find going to the Gamma Quadrant not using the wormhole, or the Barzan wormhole reaching on the galaxy's satellite arms in TNG. And yes Voyager only saw a some small fraction of the total Delta Quadrant but it seems to be the Borg would be the dominant power, only I think the Voth would have been able to hold them conventionally. Of course there other powers the Vaadvuar and that culture in the novelverse being examples but I imagine they all exist in the Borg's shadow.
19% is quite a lot. That's almost a whole quadrant. And this all before the discovery of the passageway, and Voyager's trip. Also, we could take that to mean explored 19%. Charted/explored are synonymous. But certainly they've mapped most of the galaxy by this time. They've been sending out deep space probes for 300 years.
As a side note, the universe feels Vast, dark, mysterious, wondrous in TOS, And early TNG. After late TNG and DS9, The "Alpha Quadrant" feels a lot smaller, more predictable, no longer mysterious. By the 29th century, i get the distinct impression that The Milky Way has been tamed, and the Federation holds a place of prominence. And humans hold a definitive place of prominence in the Federation.