USS Voyager moving through fluidic space at impulse
The view from inside the ship
Fluidic space is a realm located in another dimension, consisting entirely of matter in a fluid state, rather than a vacuum, and accessible only through the creation of an interdimensional rift using quantum singularities. The fluidic realm has no stars or planets, and the only lifeform known to exist there is Species 8472.
In 2373, the Borg discovered the existence of fluidic space and launched an invasion of the realm, with the intention of assimilating all lifeforms within. This initial attack was easily driven off by Species 8472, precipitating a full-scale war. The USS Voyager was later forced into fluidic space, where it engaged Species 8472 with modified nanoprobe weaponry. (VOY: "Scorpion", "Scorpion, Part II")
In 2376, the rebel Borg drone Axum was posted on a Borg ship near an intersection between fluidic space and normal space in the Beta Quadrant. During the destruction of Unimatrix Zero, he vowed to contact Species 8472 in an attempt to enlist their help in fighting forces loyal to the Borg Queen. (VOY: "Unimatrix Zero, Part II")
Apocrypha
Fluidic space is seen in the game Star Trek: Armada II, in which a temporary Borg-Federation alliance is formed to drive off Species 8472 and permanently seal an interdimensional rift in the Alpha Quadrant. In the game the region is portrayed as being impossible to form a stable warp field and lacked any resources normally found in other maps.