Deep Dark Galaxy

Deep Dark Galaxy is a galaxy in Super Mario Galaxy. It is accessible from the Garden and is a ghost-themed beach galaxy, similar to the Bigmouth Galaxy and the Bonefin Galaxy. Its large rocky main planet includes a beach and an underwater cave that leads to a ghost ship. There are only a few other small planets aside from this 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 is found here and have installed a cannon, two lawn chairs and an umbrella. In the mission Bubble Blastoff, the cannon is blocked by a bubble-shooting Undergrunt Gunner, which must be defeated in order to access it. Other features include several palm trees and a few coconuts. Past the gate with the torches, there is a cave leading to a lake that has many large moving blocks situated around it. 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. It has a Sling Star on the top and a Fire Flower on the bottom. There is a Rainbow Star floating in front of the planet that Mario can use to defeat 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, complete with the clouds in the background disappearing so that it looks identical to Gateway's. It also has three Goombas 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 disappears 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.

Due to a glitch, 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. Using first-person mode, 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 purple box-shaped planet with golden tubes coming out of its sides. It is located behind the main planet. Mario enters this planet by launching through an opening on the bottom. Inside, there is a Gravity Arrow on the wall opposite to the camera; this arrow rotates 90 degrees clockwise when spun, causing the gravity to change. There is also a Ground Urchin and a Boo containing a Power Star here. The ceiling is covered with water so that it rains inside the box. Mario must defeat the Boo to get its star, which can be done by destroying the crystal on the right wall of the room and luring it into the beam of 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 kind is visible in the distance in 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.