The Borg became aware of fluidic space in 2373, and utilized a series of quantum singularities as interdimensional rifts in order to access the realm. The Borg, however, found that Species 8472 was resistant to assimilation, and a war between the two powers erupted, in which Species 8472 began an invasion of the Milky Way Galaxy via the singularities. The ensuing six-month conflict cost the lives of millions of drones, thousands of Borg vessels, and dozens of star systems.
When the USS Voyager came upon Borg space, they formed an alliance with the Collective, wherein they would develop a means by which Species 8472 could be successfully combatted, in exchange for safe passage through their space. Seven of Nine would later modify Voyager's navigational deflector to emit a resonant graviton beam, forming another rift to fluidic space. While there, Voyager fired a series of biomolecular warheads, destroying a number of Species 8472's bioships. (VOY: "Scorpion", "Scorpion, Part II")
As of 2377, Borg vessels were actively patrolling the borders between the Milky Way Galaxy and fluidic space. Axum's vessel was one such Borg ship, stationed in a remote area of the Beta Quadrant. When the Borg resistance movement began, Axum entertained the idea of making contact with Species 8472, and soliciting their assistance in combating the Collective. (VOY: "Unimatrix Zero, Part II")
In Star Trek: Armada II, fluidic space is the setting of the last level of the game. The realm is invaded by a combined Federation-Borg armada to put an end to a renewed Species 8472 invasion of the Milky Way Galaxy. The region is portrayed as not having any kind of natural resources and it is not possible to travel at warp speeds while inside the realm.