In ancient times, mariners would tell tales of giant tentacles writhing up from the deep, pulling men off the decks of ships to grim deaths in the belly of unseen horrors. We laughed at such things, wise as we were. We looked at the old maps, with their 'here be monsters' warnings, and fanciful drawings of serpents and terrible whales. It seems that no matter how advanced we become, how impressive our ships are, there are still going to be monsters in the deep. We traded deep waters for deep space, and despite centuries of being told how barren and empty space was, we found monsters.
Exosomes are most commonly encountered in a colonial form, usually a long flexible tube. This is often described as looking like a tentacle, if the open ends and hollow center are not noticed. Superficially similar to pyrosomes and zooids in terrestrial oceans, exosomes are found in deep space, well beyond the orbit of Jupiter. The creatures are easy to miss, they are not bioluminescent, and their soft gelatinous bodies do not appear well on sensors and scanners. More often, they appear as blurry areas, or are overlooked as background noise on scans.
It was a tentacle, a big ass tentacle, space squids man, space squids! It was stealthy, couldn't pick it up on the DRADIS. I could see it, looking out through the periscope, through the optical filters. It was there, estimated size was well over ten kilometers long, big enough that it could wrap around our little frigate a half dozen times, could have cracked the hull, spaced all of us, and then eaten us at it's leisure.
The exosome is a manifestation of simple space life, lumped together by terrestrial science as space planktonoids. The exosome is a colonial structure, composed of hundreds, thousands, or even tens of thousands of individual 'planktonoids'. The creatures have elected to form a colonial body, allowing them to more efficiently feed on other planktonoids, resist predation, and to be more mobile than individually possible.
The Grimm-Olduvai index classifies exosomes as cosmozoa salps, and as least dangerous level creatures. Unlike aphasic planktonoids, exosomes are fully existent in three dimensions. They do not have any ability to harm humans, but their biomass can cause damage to ships that hit them, as well as fouling sensors, antenna, and other delicate structures. This isn't a factor of the toughness of the exosome, but more a factor of the high velocity of ships moving in deep system maneuvers. Exosomes are mindless, and if exposed to a hostile stimulus, such as engine exhaust, energy weapons, or high energy scanners, they can form a sort of pulse jet, allowing them to move slowly away from the stimulus. Despite the fantastic claims of spacers, exosomes are harmless to humanity.
The Real Deal
The Exosome is based off of the real world pyrosome, a colonial plankton found in the ocean, where they form semi-mobile tubes up to several meters long and as big around as a scuba diver.