In "The Best of Both Worlds, Part II," the Enterprise fires a burst of antimatter at the Borg Cube, apparently to no major effect. Why didn't the antimatter tear through the Cube?
The Borg had already adapted to the potential threat of the antimatter beam because they knew it was coming and altered their shields accordingly.
- More importantly, the antimatter spread was merely a diversion to allow the Enterprise crew to recover Picard.
In the opening sequence, Locutus of Borg (Jean Luc Picard) quietly reports:
"The knowledge and experience of the Human, Picard, is part of us now. It has prepared us for all possible course of action. Your resistance is hopeless, Number 1."
Once Captain Picard was absorbed into the Collective, all of his knowledge, his battle strategies, anything the Federation could know or do, became available to them. This would allow them to know exactly how to defend against any conventional attack.
Borg ships are ridiculously powerful, with engines and powerplants scattered throughout the cube. With the right information regarding frequencies, energy profiles and technological specification, they would be invincible against an enemy whom they have absorbed into the collective.