The Gekli, also known as Great Ones by the Hylasa, were large living cosmozoans existing entirely in the vacuum of space. In 2367, the crew of the USS Enterprise-D first discovered one creature orbiting the seventh planet of the previously uncharted Alpha Omicron system but killed accidentaly when minimal-powered phasers were used in self-defense against the creature's aggressiveness. The reason for the aggressiveness was discovered when scans by Lieutenant Commander Data soon revealed that this creature was about give birth. Then the crew of Enterprise assisting the birth, and the entity attached itself to the Enterprise thinking of it as its mother and draining its energy. Ultimately the creature was "weaned" and left the ship, and the newborn baby welcomed by other entities, and they all proceeded back into the asteroid field together. (TNG episode: "Galaxy's Child")
Odo once encountered the entities in the Alpha Omicron system. With his Changeling abilities, Odo was able to shape-shift into the entities for travel through space. (DS9 eBook: Rules of Accusation)
In 25th century it was discovered that they were native to fluidic space. These living creatures were capable of travelling through space under their own power and were able to move at faster-than light speeds once they built up enough energy for this task. Their existence in fluidic space meant that they were often hunted by the Undine who herded them and attempted to control these large beings.
A Great One's internal features allowed for the presence of other races which included another fluidic space species known as the Hylasa. This symbiosis was a beneficial one as the Gekli provided transportation along with access to food whilst the Hylasa provided company as well as protection to the young of the Great One. Being deprived of these symbionts led to a Gekli suffering from loneliness that eventually led to their death.
Their senses were acute enough to allow them to find food for both themselves and their passengers. Furthermore, they were able to track and locate a quantum singularity to allow access to other dimensions such as the one populated by the Milky Way Galaxy. In 2409, contact was made by Starfleet/Klingon Defense Force with the Gekli along with the Hylasa during attempted peace negotiations with the Undine. The Gekli were responsible for allowing the first contact crew to return back to their own dimension. (STO mission: "Fluid Dynamics")