A gaseous anomaly is a type of interstellar phenomena. In the early 2290s decade, Starfleet were keen to investigate gaseous anomalies and equipped all starships with specialized probes that could monitor them. They also dispatched the USS Excelsior on a three-year mission to chart anomalies in the Beta Quadrant in 2290. (TOS movie, novelization & comic adaptation: The Undiscovered Country)