Redezra opens the large book of Borg Canon...
Oookay, lets see here...
So it's been known for some time that the Borg's primary target at the moment is the species in the Alpha quadrant (as a result of Q Who), and as the middle of the galaxy is somehow a magic doom sphere apart from a certain movie, the Borg are following the path of least resistance. In entirely the other direction to the Dominion. So I'd say they don't know about the Dominion yet.
The Borg are also focusing a lot of attention on assimilating species in the way, and species 8472, who are also immune to assimilation. It is this "immunity" I wish to discuss.
In the primary timeline, post Romulan Catastrophe, and I refer you to the word-of-god canonical Star Trek Online storyline, the Borg rock up to assimilate the Alpha Quadrant, and again, seem to not be there when the Dominion show up again. These two seem to be decidedly good at avoiding eachother. However, the Borg succeed in assimilating species 8472, even if the "success" is experimental and only with a couple of specimens. What this demonstrates is that nothing is immune to the relentless will and sheer intellect of the Borg Hive.
Secondly, the Dominion, for all their might, and all their navies and superior strategies and etc etc etc... were beaten by the combined forces of the alpha quadrant. The Borg nearly annihilated the entire Federation fleet with a single cube. And then proceeded to almost completely obliterate the Federation itself with another. The Borg are not bogged down by standard tactics, are immensely powerful, have the most advanced drive and shielding technology of all major races, are capable of disabling multiple ships in seconds, and can only be taken down reliably by people with prior knowledge of Borg systems. Add to this their capacity to react and make themselves immune to any weapon, and all up, they are the most formidable force in the galaxy bar none.
So, assuming that the Borg ever met the Dominion, and decided that they were a priority, the Dominion is screwed. Their fleets are inferior to ones that are swept aside with impunity by single Borg ships, their tactics mean nothing to something that quite happily employs temporal plot hax and feeds itself on their enemies corpses, and the Founders are only temporarily immune to Borg assimilation.
In short: Resistance is Futile.