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 also Magikoopas near it. The thing that makes it "spooky" is that there is an underwater cavern (It's rather deep and dark, giving the Galaxy its name) filled with ghost ships. There are very few planets aside from the big, main one. Guppy makes his second appearance here.

Beach Planet
This is the main planet of the galaxy. It has a small beach with a few crabs, 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. 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, Gringills, Jellyfish, Urchins, and some sort of Angler Fish. 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.

Wooden Planet
This is a small planet with tons of Electrogoombas 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 Electrogoombas.

Gateway Galaxy Planet
This looks a almost identical to the main planet in the Gateway Galaxy but on a much smaller scale. It has three Goombas living on it. There is a gold screw where the castle should be and, when spun will cause the planet to lose the air inside of it and release a ring of coins. It will shrink until it can shrink no more and then it will explode, sending Mario to the Wooden Planet.

Cheep-Cheep Planet
This 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 proceed.

Watermelon Planet
This is a big glass planet with three tennis balls and a watermelon inside it. There is a shock emitter that sends shockwaves around the planet. Mario must ground pound the tennis ball into the watermelon to get the star.

Boo Box
This 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 Boo 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.

Enemies

 * Boos
 * Bloopers
 * Electrogoombas
 * Cheep-Cheeps
 * Crabs
 * Firebars
 * Fire Shooters
 * Magikoopas
 * Goombas
 * Gringills
 * Jellyfish
 * Urchins
 * Water Shooters
 * Underwater Mines

The Underground Ghost Ship
Mario begins on an island of some sort with the Toad Brigade already exploring it (They are on a "vacation" although they claim they are searching for Power Stars). The player must jump into the cannon on the center of the island. The player must make sure to point the cursor at the Starman which will give Mario invincibility. When Mario lands on the planet, he needs a Fire Flower to become Fire Mario and he should use the launch star to go back to the island. The player should light both urns in one try. Once Mario opened the gate, he must travel into the cave and he must dive into the body of water in front of him. Mario must dive into the water and the player will find a lever. Mario should do a Spin to open the gate. The player must swim to the surface. Mario will find a ghost ship surrounded by Boos. The player should go on the ghost ship to fight Kamella. Once Mario defeated Kamella, the player gets a Power Star.

Bubble Blastoff
Mario must defeat Undergrunt Gunner by spinning coconuts into him or ground pounding the cockpit. Then he will have to get the fire flower from the wooden planet, like the first mission and light three torches to make an Ice Flower appear, go across the lake and go to an area full of obstacles, then Mario will have to use a Water Shooter to get to the Cheep-Cheep planet, then go to a transparent planet that has a watermelon and tennis balls inside it. Mario must ground pound the tennis balls into the watermelon 3 times to make it explode, and reveal a Power Star that was inside it. However, there are shocking force fields that are generated to make this task hard.

Guppy and the Underground Lake
The player will have to grab a shell and get to Guppy in the lake. Once again, the player needs to go through his eight rings to get the Power Star.

Ghost Ship Daredevil Run
Mario starts off in the scene where he lands in front of the ghost ship that is surrounded by Boos. The player will have to go on the ghost ship to fight Kamella. Every now and then Kamella throws a green shell at Mario, which he must use to attack her. The best way to defeat Kamella is to take out the Magikoopas and then wait for her to throw a shell at Mario. Once Kamella is defeated, the player gets a Power Star. However, the mission is made much harder by the fact that you only have 1 health unit to last the whole fight.

Plunder the Purple Coins
The level immediately starts at the Underground Ghost Ship. Once the player collects all 100 coins variously spread out, Mario will earn a Power Star.

Boo in a Box
The player needs to start the Underground Ghost Ship, Bubble Blastoff or Guppy and the Underground Lake. The player will have to go to the bottom of the underground lake and Mario will have to grab a shell. Mario will have to throw the shell at several mines surrounding half of a ship. The ship will explode, revealing a Launch Star. The player will have to use the Launch Star that leads Mario to a box-like planet. In the planet, Mario will have to Spin next to a arrow in the planet to change the gravity. The player will have to break a crystal which will make light go into the planet. The player will have to lead the Boo into the light to make it give up the Power Star it has. Mario will have to get the star by hitting the arrow once more.

Trivia

 * This galaxy is similarly named to an area in the SNES game EarthBound, called the Deep Darkness. A similar situation applies to the Dusty Dune Galaxy.
 * The Underground room, accessible via a warp pipe at the top of the main planet is viewable from the beach. This is easier to view on the third star, as the cannon is missing. Standing where the cannon once was, look just right of the smaller version of the starting planet, there will be a weird box like object at a very far distance. Flying there via hacking reveals the object to actually be the room the player need a warp pipe to access, but the purple background is only viewable when Mario or Luigi is really close to the room encircling the room as a sphere. Its revealed that all galaxies with a underground room have this kind of planet in the sky, with some more well hidden then others.
 * 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 and will eventually lead 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 permenantely be trapped there, as Kamella won't appear. Even if Mario/Luigi dies there, he will still respawn in the Ghost ship area.