This gaseous living creature is about 800 thousand kilometers across, and half of that in depth; and made out of strange combinations of matter and energy, including elements unknown to the periodic table. In spite of its irregular form and non-solid structure, the cloud is far from internally homogeneous and actually has a recognizable anatomy, with specific locations and structures analogous to an early digestive tract, intestines and a brain, the later of which works based on electrical activity.
The cloud is able to move at interstellar speed, seeking star, planets and other chunks of matter to consume. It captures food by extending strings made of a previously unknown type of energy which generate a powerful force of attraction. According to Spock, the strings resemble ambiplasma (which is plasma formed by matter-antimatter combinations). The first portion of the cloud's digestive system contains several blobs of antimatter which act to break down large portions of the matter engulfed. Deeper on there are antimatter structures analogous to intestinal villi. The cosmic cloud is able to absorb the energy of a phaser beam, rendering it unharmable by this type of weapon.
It is unclear how the cloud is able to generate antimatter within itself, and how it maintains its matter-antimatter structure stable. Presumably, part of the antimatter could come from the engines of the starships it consumes, but that would hardly be enough for the quantity needed to dissolve entire planets into energy.
Prior to its encounter with the Enterprise, the cloud was unaware of the existence of humanoid lifeforms. Using the ship's computer to translate and amplify his thoughts, the Vulcan commander Spock was able to perform a mind meld with the creature. The intelligent cloud conceded to spare the inhabited planet Mantilles, which it was about to consume, and returned to its region of origin.
- Star Trek: The Animated Series, s01e03 "One of Our Planets is Missing"