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. However, the main planet in it has a beach with many Toads and crab-like enemies. There are also Magikoopas near it. The thing that makes it "spooky" is that there is an underwater cavern (It's rather 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.

Enemies

 * Boos
 * Bloopers
 * Electrogoombas
 * Cheep-Cheeps
 * Magikoopas
 * Goombas

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 Star 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 star spinning coconuts into him. Then, the player must go into the cannon to blast-off to a metal base far at the side. However, Mario must time it correctly, to hit the wall, or Mario will blast off into space. After blasting off to the base, the player will go through it, and Mario will have to use a Launch Star to a transparent planet that has a melon and tennis balls inside it. Mario must ground pound the tennis balls into the melon 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.

Boo in a Box
The player needs to start the Underground Ghost Ship mission. 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 Star 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.

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.

Trivia

 * The first planet of the Gateway Galaxy makes a cameo appearance here. It can be seen from the planet full of Electrogoombas. It can be visited via the cannon and a stump switch will make the planet shrink, revealing a ring of coins.
 * 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, accesible 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 wierd 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.
 * There is a launch star high above the galaxy for unknown reasons. Its possible only through hacking and it doesn't take Mario any where. So he will eventual die of falling.