USS Voyager moving through fluidic space at impulse
The view from inside the ship
Fluidic space is a realm outside of the Milky Way Galaxy that was home to Species 8472. Very little was known about the region, but it was described by Lieutenant B'Elanna Torres of the Federation starship USS Voyager to be not space, but matter. There were no known stars or planets located in fluidic space and the region could only be accessed by quantum singularities.
In 2373, the Borg became aware of fluidic space and attempted to invade the realm and assimilate Species 8472. Unfortunately for the Borg, 8472 proved to be resistant to assimilation and quickly destroyed the Borg invasion force. Shortly afterwords, Species 8472 invaded the Delta Quadrant and retaliated against the Borg. The ensuing six month conflict cost the lives of millions of Borg drones, destroyed thousands of starships, and dozens of star systems. The war was brought to an end thanks to the efforts of the USS Voyager. (VOY: "Scorpion, Part II")
By 2377, the Borg were actively patrolling boundaries between the Milky Way Galaxy and Fludic Space. (VOY: "Unimatrix Zero, Part II")
Apocrypha
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.