It is another dimension apart from our own. In fluidic space, space is literally a fluid. In this dimension, a hostile species (Species 8472) lived. The Borg opened a rift in space using a deflector dish, trying to conquer Fluidic Space. However, species 8472 was extremely resistent to the Borg's efforts to conquer them, and ended up fighting and prevailing against the Borg in normal space. The crew of the U.S.S. Voyager helped the Borg defeat Species 8472 and send them limping back into Fluidic Space. In return, the Borg allowed Voyager safe passage through their space for a time.