Fluidic space is an extra-dimensional realm filled with a form of organic fluid and containing no stars or other celestial bodies. Its sole indigenous lifeforms were Species 8472. Objects moving through fluidic space generated pressure waves detectable by Species 8472.
The Borg became aware of fluidic space in 2373 and utilized a series of quantum singularities as interdimensional rifts to access the realm. However, Species 8472 proved impossible to assimilate and invaded the galaxy in return. The ensuing conflict resulted in Species 8472 destroying hundreds of Borg planets and millions of drones.
When the USS Voyager arrived at Borg space, they allied with the Collective, offering modified nanoprobes that could destroy Species 8472 cells in exchange for safe passage. After Commander Chakotay decided to terminate the alliance, the Collective ordered Seven of Nine to take Voyager into fluidic space to force a confrontation. Voyager destroyed several Species 8472 bio-ships with nanoprobe-armed bio-molecular warheads, causing the remainder of their forces to retreat to fluidic space (VOY: "Scorpion", "Scorpion, Part II", "In the Flesh")
When Voyager locked down after encountering the Omega phenomenon later that year, Ensign Hickman speculated the cause to be a new Species 8472 incursion from fluidic space. (VOY: "The Omega Directive")
Axum was stationed aboard a Borg scout ship patrolling a border with fluidic space in a remote sector of the Beta Quadrant. After he was liberated from the Collective in 2377, Axum entertained the idea of contacting Species 8472 and soliciting their assistance. (VOY: "Unimatrix Zero, Part II")
Several missions in Star Trek Online are set in fluidic space, which is also revealed to be the native domain of the spaceborne lifeform from TNG: "Galaxy's Child". The realm is shown to be heterogenous with clusters of organic "land". Warp and transwarp are not possible within fluidic space.