Fluidic space was an extra-dimensional realm that was filled with an organic liquid medium rather than the open space of the Milky Way galaxy's dimension. Fluidic space was the home of Species 8472, and was accessible from the regular universe via quantum singularities. It was not possible to use either warp or transwarp drives within fluidic space. (VOY episode: "Scorpion"; TNG video game: Armada II)
Whilst it was believed that the Undine were the only inhabitants of this realm, this was not entirely true as in the beginning of the 25th century it was discovered that at least one Cosmozoan lifeform known as the Gekli inhabited fluidic space. Furthermore, these massive organisms had humanoid symbionts within them known as the Hylasa. In addition, whilst the majority of fluidic space was filled with liquid medium, it was known that there were large organic-based land mass clusters of matter similar to coral reefs that were a rough equivalent of planets within this extradimensional realm. (STO mission: "Fluid Dynamics")
In 2373, fluidic space was discovered by the Borg Collective, who launched an invasion into the realm hoping to assimilate the highly biologically-developed Species 8472. The Borg's attempts proved futile and in response Species 8472 launched a counter attack, aiming to wipe out life in the galaxy, starting with the Borg.
After several months of heavy losses the Borg were contacted by the crew of the USS Voyager who agreed to help them develop a weapon to fight Species 8472 in exchange for safe passage through Borg space. The Borg agreed, and later in the development of the weapon forced Voyager to enter fluidic space to test the weapon on Species 8472 vessels.
In 2377, Species 8472 launched a new offensive on the Milky Way. They created three large rifts through which to enter the galaxy and also developed the fluidic gate, an organic structure which they used to make smaller quantum singularities to allow additional reinforcements to travel through from fluidic space.
Species 8472's renewed hostilities eventually prompted the Borg and the Federation to form an alliance to fight back. The combined forces of the Borg and Federation successfully captured the Alpha Quadrant end of one of the three rifts and then launched an offensive into fluidic space to destroy the Rift Maker, a large Species 8472 organism that was holding the three rifts open.
In an effort to end further hostilities with the Undine and thus remove the threat posed by them in order to focus on the Borg; Starfleet decided to send a diplomatic delegation into fluidic space by way of a quantum singularity found in the Zemet system. After entry into this extradimensional realm, the Starfleet force tried and failed to dissuade the Undine from further attacks. However, they did make first contact with another native race to this extradimensional realm known as the Hylasa. (STO mission: "Fluid Dynamics")
Kilana was abducted with her Jem'Hadar by the Caretaker. She attempted to rejoin the Dominion by returning to the Gamma Quadrant. In an alternate reality, she was found by Kathryn Janeway. After a deal with the Voth went wrong, she found herself trapped in fluidic space. (Star Trek: Myriad Universes novel: Places of Exile)