Fluidic space
Fluidic space was 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  arrived at Borg space, they allied with the Borg Collective, offering modified nanoprobes that could destroy Species 8472 s 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. ()

In 2374, a member of Species 8472 who had been stranded in the Delta Quadrant boarded Voyager and unsuccessfully tried to use the deflector to open a quantum singularity. ()

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. ()

After the Voyager crew reached a peaceful settlement with Species 8472 in 2375, the Species 8472 within the terraspheres returned to fluidic space. ()

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. ()

In 2378, the bored  suggested to Kathryn Janeway that they could enter fluidic space and fight Species 8472. ()

Apocrypha 
The last level of Star Trek: Armada II is set in fluidic space. The realm is portrayed as lacking any kind of natural resources and it is not possible to travel at warp speed inside.

A few missions in Star Trek Online involve the player traveling to fluidic space. The realm is shown to contain clusters of coral-like structures the size of celestial bodies.

Star Trek: Alien Domain takes place in fluidic space, which is portrayed with celestial bodies and wormholes.

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de:Fluider Raum
ja:流動空間
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