The matter-energy cloud or cosmic cloud was a non-corporeal cloud creature irregular in shape, measuring some 800,000 kilometers across, and about 400,000 kilometers in depth – or, according to Arex, twice the diameters of Jupiter, Saturn and Neptune together. It was opined to come from outside the galaxy.
In order to survive, the cloud lived on the energy it converted of the matter it consumed. This process involved began with the use of tendril-like streamers, that were composed of a combination of koinoenergy (almost an ambiplasma) with an unusually powerful attraction force to gather matter. It then used blobs of highly charged gaseous antimatter, in much the same way the humanoid body uses macromorphase enzymes, to break down the matter it engulfed in order to consume it.
Encounter with the Federation
In 2269, this huge cosmic cloud was reported moving into the outer fringe of the Milky Way Galaxy. Starfleet Command sent the USS Enterprise to investigate the phenomenon, as nothing like it had ever been seen before.
On stardate 5371.3, the Enterprise located the cloud in the Pallas 14 system and intended to intercept it near Alondra. However, before it was able to intercept, the cloud engulfed Alondra, and proceeded to break the planet up, consume it, and proceed to change course towards Mantilles.
Spock theorized that the creature may perhaps instinctively seek out planets to consume in much the same way an amoeba seeks out food. Not knowing if the creature had any selective ability in its search for matter to consume, he warned that it may also seek out stars to consume as well, and may become a threat to every planet in the galaxy.
As the Enterprise approached the cloud, the cloud grabbed hold of the starship with its strings and began to engulf it. Captain Kirk ordered that the ship fire full phasers at the cloud mass; however, this proved ineffective, as the cloud simply absorbed the phaser's energy.
The ship was then approached from all directions by a number of the clouds antimatter objects, which began to drain the ship's shields. These antimatter objects would eventually be deterred by the use of antimatter charges.
After observing this series of events, Spock determined that this cloud, and its processes, resemble that of a living being. This led Dr. McCoy to warn that if that was the case, and if the shields did fail, then the antimatter objects surrounding the ship would corrode the hull, and everyone on board would be broken down into "nice digestible particles." Noting that the blobs performed the same function as teeth, as they break up the larger chunks of matter for digestion.
In an attempt to escape the cloud, Kirk intended to head for the second opening discovered at the other end of the cloud, meanwhile hoping that if it did have a stomach, that they could give it enough indigestion to turn away from Mantilles.
Upon reaching the central core of the cloud, the Enterprise encountered large explosions that violently shake the ship, leading McCoy to liken the region to the small intestine. McCoy noted that the small extrusions growing from the wall appeared to be villi, and based on sensor data, were determined to be composed of antimatter.
Initially, Spock likened the behavioral patterns of the cloud to "a huge bull, grazing here and there, in the pasture of the universe." He later reanalyzed that assessment after noticing that the cloud made the seemingly intelligent decision to change course from Alondra to Mantilles, indicating that it had a brain. This was supported by the considerable electrical activity near the entryway at the top of the core.
In considering the lives of the inhabitants of Mantilles, Kirk intended to use the photon torpedoes to destroy the brain. Spock and Uhura were given the job of locating target points in the brain, however, once analysis was complete, Spock noted that the size of the cloud's brain was so immense, that entire offensive armament of the Enterprise could not guarantee its destruction.
Kirk then considered using a Vulcan mind touch to contact the cloud. Although Spock could not make direct contact with the cloud, he suggested that it could be done by directing the ship's sensors onto the cloud's synaptic electric impulses, the input could be transferred through the ship's computer, then route it through the universal translator, and the ships audio system.
Upon contacting the cloud, Spock explained to it that he within the cloud. The cloud did not understand how another being could be inside it. Spock explained that he was on a starship and that the cloud was endangering many like him on the planets that the cloud consumes. He warned the cloud not to consume Mantilles, and to sense it closely. Spock proceeded to enter the mind of the cloud, and in exchange, the cloud entered Spock's body, and after viewing several images from the library computer of Earth, the cloud comprehended what it did and stopped its advance on Mantilles, and was successfully convinced by Spock to return to its origin. (TAS: "One of Our Planets Is Missing")