Deep Dark Galaxy

Deep Dark Galaxy is a ghost-type galaxy in Super Mario Galaxy, like the Boo's Boneyard Galaxy and the Ghostly Galaxy. The main planet in it has a beach with many Toads and Crabs. There are very few planets aside from the big, main one. Guppy makes his second appearance here.

Ghost Ship Cave Planet
The Ghost Ship Cave Planet is the main planet of the galaxy. It has a small beach with a few Crabbers (one of which will give a 1-Up Mushroom when defeated), one Starbag and, in the water, a chest with a 1-Up Mushroom. The Toad Brigade often stop here and have installed a cannon, two lawn chairs and an umbrella. In the mission Bubble Blastoff, the cannon is under control by a water Undergrunt Gunner, which must be defeated to gain access to the cannon. Other features include several palm trees and a few coconuts. On the other side of the planet there is a cave with many strange machines situated around an underground lake. The lake takes up much of the planet and is very deep. At the bottom there is a wrecked ship and two rocks shaped like stars. There is a variety of sea life in the lake, including Bloopers, a Gringill, Preying Mantas, Urchins, Mines and a Jellyfish. There is a passageway in the lake that surfaces into a cave, where there is a ghost ship inhabited by several Boos. Kamella is fought on this ship twice. If Mario manages to catch crabs, he might get a 1-UP.

Relay Planet E
Relay Planet E is a small planet with many Octoguys living on it. It has a Sling Star on the top and a Fire Flower on the bottom. There is a Rainbow Star right in front of the planet that Mario can use to get rid of the Octoguys.

Scale Down Relay Planet
The Scale Down Relay Planet looks almost identical to the main planet in the Gateway Galaxy but on a much smaller scale. It also has three Goombas living on it. In addition, there is a small gold screw in the ground where the castle would normally be. When Mario spins this screw, the air inside the planet will start to leak out and the planet will shrink as a result. As it gradually gets smaller and smaller, a large ring of coins encircling the planet will be revealed. It will continue to shrink until it implodes upon itself with a small puff of smoke, after which Mario falls back to the Relay Planet E. This planet can only be accessed by shooting Mario out of a cannon at it from the starting planet.

The player can enter the house with the blue roof. To do so, the player should crouch and enter the door of the house (achieved by holding down and walking into the doorway). Mario/Luigi should now be inside the house. The player can can see the basic virtual design of the house as well as the coins that are hidden inside the planet. The house's walls aren't solid from the inside, so to leave the house, Mario/Luigi simply has to walk out.

Cheep Cheep Gambol Planet
Cheep Cheep Gambol Planet is a tiny planet with some Cheep-Cheeps bouncing around it. It has a grass and water area with wooden plugs. Mario must pound the plugs to activate a Water Shooter, which launches him to the Marble Planet.

Marble Planet
The Marble Planet is a big glass planet with three tennis balls and a watermelon inside it. There is also a Nut-like Sentry Beam here that sends shockwaves around the planet. Mario must avoid the shockwaves and Ground Pound the tennis balls into the watermelon to get the Power Star.

Boo Room Planet
Boo Room Planet is a large box to the side of the main planet. It is a large purple box with tubes coming out of it. There is a Boo containing a Power Star and an Urchin inside the box. The box's roof is made of water so it rains inside the box; Mario must destroy the Boo to get its star. This is the area for Deep Dark Galaxy's secret Power Star.

Trivia

 * There is an inaccessible small box planet high above the galaxy (viewable from other planets). Hacking reveals that this is actually the underground room, accessible via a warp pipe at the top of the main planet. This appears in any galaxy which includes an underground bonus room.
 * For unknown reasons, there is a Launch Star high above the galaxy. It is only possible to get to the Launch Star by hacking, and it will only shoot Mario or Luigi out into space with nothing to land on. This will make them fall to their death. When the level reloads, they will start out in space and die right afterwards which eventually leads to a Game Over.
 * Mario/Luigi can reach the Underground Ghost Ship directly by firing the cannon above the rocks. It is also possible in Mission 2. If Mario/Luigi reaches the area in Mission 2, they will be permanently trapped there, as Kamella won't appear. Even if Mario/Luigi dies there, he will still re-spawn in the Ghost ship area.