To anyone who may question where the mansion in Ghostly Galaxy came from, the answer does come from thinking about it. The simplest answer is that the Boos found in the level made the mansion like they created Luigi's Mansion.
The levels are too small to be galaxies, or maybe you're simply visiting the part of the galaxy that has power stars.
This can especially be the case when you consider how many levels, most notably the Comet Observatory, are floating over a gigantic celestial body.
If you do two games, one with Mario and one with Luigi, Luigi actually gets more stars than Mario. In the main quest, Luigi finds 3 stars to help Mario, bringing the total stars Mario finds down to 118, not counting all the stars the Toad Brigade found. In Luigi's quest, you play as Luigi and have another Luigi show up, thus bringing the total stars Luigi finds in that game to 121; add that with the total from the last quest, and Luigi's grand star total is 124 to Mario's 118.
Ever wonder why the first characters you see in space are rabbits? A common Japanese belief is that if you stare at the moon, you will see the shape of a bunny, hence why space bunnies in this game
And then there's the Chinese legend of bunnies on the moon, as well as the image of a bunny on the moon's visible surface.
Cosmic Luigi has the same voice clips as Cosmic Mario. One wonders why......until one realizes that they are controlled by the same Cosmic Comet......
While playing as Luigi, if you grab the Boo mushroom, you'd see that in his Boo form, Luigi looks quite uncomfortable. Luigi's afraid of ghosts. Therefore, he's actually scaring himself while in that form.
The repeated sequence of notes in the Ghostly Galaxy theme (the one that starts the song and continues to play throughout it) is a D-E-A note sequence. Put the first note of the next sequence on it and it spells D-E-A-D. Seems all the more fitting that it plays in a haunted house level.
The Garden is easily the most mysterious location in the Comet Observatory and it's not exactly made clear why it exists as the highest dome in the observatory. Young Rosalina expressed how she wanted to return to "the house by the hill", and often refers to said hill. The Garden could potentially be Rosalina's attempt at recreating the place on her planet that means the most to her; the hill.
Whenever you reclaim a Grand Star, notice how more and more Lumas will appear in the observatory grounds. Now think of how many of them were inside the observation domes when Bowser attacked locked inside, trapped in pitch darkness all that time until the power was restored and they could escape. And these are little children we're talking about.