Fluidic space was an alien dimension which Species 8472 called its home. It contained no planets or stars or even space, instead it was covered in a liquid substance.
It was discovered by the Borg Collective who invaded this realm in order to add the biological perfection of Species 8472 to the Borg. However, the Collective underestimated the power of Species 8472 which started a war between the two sides. Species 8472 entered the Delta Quadrant through Quantum singularitys that served as a gateway from Fluidic Space into normal space. (VOY episode: "Scorpion")
After the war, Species 8472 made use of organic gateways of their own design to open a gateway into Fluidic space in order to summon their larger bio-ships. Furthermore, during their invasion of the Alpha and Beta Quadrant, Species 8472 made use of a specially bred organism to maintain the numerous quantum singularities that allowed their invasion forces into the Milky Way galaxy. (TNG video game: Armada II)
