Space-borne anomaly encountered by the Prometheus on a "pit stop" during its voyage back to Earth from Tagrea. Prometheus took refuge in the cloud from an assault by an alien vessel. Samantha Carter quickly discovered that the compounds in the cloud prohibited restart of the ship's sub-light engines, as well as the formation of a stable hyperspace window. She deduced that it was more likely made up of a group of denser gasses, rather than a typical nebulae or a gas giant.
Eventually the cloud's compounds began to corrode the hull of the ship. Carter's subconscious, while in a state of mental disarray due to a head injury, deduced the possibility that the cloud may be a lifeform, projecting the appearance of a young girl named Grace to communicate with Sam. Carter eventually worked out a way to escape from the cloud, and offered to help the hostile alien ship escape as well in return for the crew of the Prometheus, whom the aliens had abducted.
The true nature of the gas cloud remains a mystery.
Grace - Prometheus enters a gas cloud to avoid capture by a hostile alien force, and becomes trapped.