A hydrogen cloud is a nebula composed of hydrogen.

In 2372, the USS Voyager was hailed by a probe emitting a Federation signal in the Delta Quadrant. The signal originated from inside a hydrogen cloud. Voyager arrived at the hydrogen cloud, but it was later found that the Kazon were hiding there in order to overwhelm Voyager's defenses, due to the traitor Seska providing the Kazon-Nistrim Voyager's access codes. The Kazon were ultimately successful in stealing a transporter oscillator from the ship. (VOY: "Maneuvers")