otherwise they would have assimilated the entire galaxy a long time ago
What evidence do we have that this hasn't happened already? In the 24th century, the Borg are readily found in the Delta and Alpha quadrants at least, and probably in Beta as well (although we seldom hear mention of this quadrant anywhere in Trek).
However, in none of those quadrants are the Borg "in control" or otherwise a visible presence. They always lurk in the shadows, assimilate a world here, a world there, and apparently virtually never keep a world they have assimilated, preferring to live in deep space in their ships or other constructs. For all we know, the entire galaxy is already permeated by them, as evidenced by our heroes running into them basically wherever they go. (The heroes just haven't gone to Gamma much, due to the Dominion blocking the way...)
Timo Saloniemi
Well i'm with you on them assimilating different worlds all over the Galaxy, but they haven't assimilated "The Entire Galaxy"
The Borg originated in the Delta Quadrant from a canon point of view, when or how, doesn't matter, but what does is that on the 14th century it was established in VOY that they only controlled a handfull of planets in the Delta Quadrant
Later on in the 22nd Century The Borg transmitted the location of Earth to the Delta Quadrant, in the 23rd Century, The Borg destroyed the El-Aurian home planet (which was likely on the edge of the Delta Quadrant,it can't have been much later that the Enterprise B rescued the refugee ships in the Beta Quadrant)
In the mid 24th Century the Borg assimilated the Hansen's and then later the Enterprise D encountered a Borg Cube in System J-25, 2 years later the Borg attempted to Assimilate Earth's inhabitants
Basically what i'm saying is, is that the Borg have only assimilated planets within the Delta Quadrant, in other quadrants they've merely destroyed civilisations and assimilated the inhabitants and technology as to why thats unknown, but they've only ever tried to assimilate Earth on one occasion (and that was when the Enterprise E tharwted them in the past, the other time could have been like what they did with other planets, scooping their technology off the planet and assimilating the inhabitants, leaving nothing but massive craters where civilisations once were)
The Borg don't rely fully on Biological matter, therefore they would have no need for a planet, as we've seen, the Borg build giant Unimatrix' in Space
What i'm getting at is that the Borg operate from the Delta Quadrant, they move around the Galaxy choosing civilisations to assimilate, but they don't actually take over planets...if they did then the entire Alpha Quadrant would be overrun by them