Dusty Dune Galaxy is the site of an Apocalypse How.Like Atlantis, a prosperous civilization once thrived on that planet and its assorted mini-moons. They built pyramids, monuments, and even orbiting space-temples with their own gravity. Then, like Atlantis, some sort of disaster struck and and all the earth in that region started turning to quicksand, driving the inhabitants to extinction. By the time Mario visits, only a few scattered ruins and barely-functional automated defense systems are left as a testament to the builders' great achievements.
The events of the Luigi playthrough happen in Luigi's imagination.After returning from his adventure, Mario eventually gets around to telling Luigi about what happened. Later, while either dreaming or daydreaming, or possibly even during Mario's storytelling, Luigi plays out the story in his head, except he replaces Mario with himself doling out the heroics. He apparently doesn't put too much effort into making the dream believable though, since he still ends up rescuing "Luigi", there's only sparse unique dialog concerning Luigi, and Bowser considers second-banana, gets-lost-finding-stars, probably-afraid-of-his-own-shadow Luigi as much of a legitimate threat as Mario himself.
Black Doom is Rosalina's brotherAfter finding out about his sister's journey into deep space, he started out on his own journey to find her. However, the aliens he encountered were not as friendly as the Lumas. These were the Black Arms, a murderous race of conquerors. He fought tooth and nail against these beasts, but even he fell to them after a long battle. The dying leader, impressed with this human's resilience and fighting prowess, saved his life by reconstructing his body into that of a new leader of the Black Arms. Afterwards, the leader died, leaving the reconstructed human to lead as Black Doom. Since then, he lead the Black Arms in a sick parody of Rosalina's life among the Lumas. Instead of nurturing them to grow, he ordered them to kill and pillage. Instead of peace, he promoted war. And instead of birth, he believed the most in death. And when the Comet Observatory finally makes contact with the Black Comet, there is not telling what will happen when brother and sister are reunited.