Fluidic space is an extra-dimensional reality, and is filled with a form of organic fluid. It contains no stars, and no other celestial bodies.
Its sole indigenous lifeforms are Species 8472. Objects moving through fluidic space generate pressure waves that are detectable by Species 8472.
The Borg became aware of fluidic space in 2373, and utilized a series of quantum singularities and used them as interdimensional rifts in order to access this dimension.
Unfortunately for the Borg, Species 8472 was impossible to assimilate, and in retribution, invaded the part of the galaxy the Borg had been in when they breached the space.
This conflict between the two species resulted in the destruction of hundreds of Borg planets, as well as millions of drones.
When the USS Voyager found themselves in Borg space, they allied with the Borg Collective in order to fend off the deadly Species 8472.
It was a Voyager invention (along with some Borg improvements) that was able to fend off the Fluidic Space species.
Modified nanoprobes could destroy the cells of Species 8472, and the Voyager crew used these to barter their way through Borg space on their way back towards home.
Voyager was able to destroy several Species 8472 bio-ships with nanoprobe-armed bio-molecular warheads, causing the remainder of 8472 forces to retreat back to fluidic space.