Deep Dark Galaxy

Deep Dark Galaxy is a galaxy in Super Mario Galaxy. It is accessible from the Garden and is a ghost-themed galaxy, like the Boo's Boneyard Galaxy and the Ghostly Galaxy, although its theme is closer to that of an abandoned cave. The main planet in it has a beach with many Crabbers. 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 and serves as the starting planet. It has a small beach with a few Crabbers (one of which will give a 1-Up Mushroom when defeated), one Tracks 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, Jammyfish, Urchins, Mikey Mines and a Jam-o'-War. 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. In The Underground Ghost Ship, the sky above the planet is orange; in Bubble Blastoff, it is blue; and in Guppy and the Underground Lake, it is clear.

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 (complete with the galaxy's background changing to match Gateway) 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 see the basic virtual design of the house as well as the coins that are hidden inside the planet. The house's walls are not visible from the inside due to back-face culling, so to leave the house, Mario/Luigi simply has to walk out.

Pukupuku Gambol Planet
Pukupuku 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-type Ball Beamer 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.

Teresa Room Planet
The Teresa Room Planet is a large box to the side of the main planet. It is a large purple box with tubes coming out of its sides and a Gravity Arrow in the middle of one of the walls opposite to the camera; this arrow rotates 90 degrees clockwise relative to the camera, causing the gravity to change. There is an Atomic Boo that serves as a minor boss and contains a Power Star and a Ground Urchin inside the box. The box's roof is made of water so it rains inside the box; Mario must destroy the Atomic Boo to get its star. This can also be done by destroying the crystal on one side of the room and luring the Atomic Boo into the light. This is the area for Deep Dark Galaxy's secret Power Star.

This planet actually has two different models. When viewed from somewhere else, it is darker and is also spinning. However, when Mario is actually being launched toward the planet, it looks brighter and is stationary.

Trivia

 * There is an inaccessible small box planet high above the galaxy (viewable from other planets, and from the main planet in the third mission). 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, though they are typically hidden in areas that cannot be seen in normal gameplay. Another exception is in the Good Egg Galaxy, while a spherical planet of the same intent is visible in an upper room corner of the Toy Time Galaxy.
 * 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 fall forever.
 * 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 (unless the player pauses and chooses to return to the Comet Observatory), as Kamella will not appear. Even if Mario/Luigi dies there, he will still re-spawn in the ghost ship area.