Darkness on the Horizon

Darkness on the Horizon is the only mission of Bowser's Dark Matter Plant in Super Mario Galaxy. It is the second of three galaxies where Bowser is fought.

Layout
The player starts on a small platform next to dark matter and moving platforms. Those platforms take the player to a group of horizontally moving platforms the player has to cross to avoid falling into the dark matter. Afterward is a two-dimensional platforming room with many moving platforms and changes in gravity; the player has to stand on the right spot of the L-shaped platforms when the gravity changes to progress. Up a pole are moving holes of dark matter that show the player what is coming up. Several large moving platforms ahead move through dark matter holes that can cause the player to fall into the dark matter. After the moving platforms is another two-dimensional platforming challenge with moving platforms and gravity changes, though this time there is dark matter that the player can fall into. The plus-shaped platform moves through different directions of gravity, so the player needs to move accordingly to avoid falling into the dark matter and move on. At the end of the gravity chamber is a lift that takes the player to a tiled platform, and rescuing the Lumas from crystals causes stairs to appear. Once the player steps onto the circular platform, Bowser will appear and take the player to a planetoid above.

The method of defeating Bowser is the same as in the last fight: Bowser jumps up and attempts to land on Mario. If Mario is standing on a blue covering, Bowser destroys it and burn his tail on the lava underneath. Bowser then starts to run around the planet and the player has to spin into his tail when he turns around, followed by spinning again when Bowser is on his back. Each time Bowser gets flipped over, he moves faster along the planet. When flipping him over for the third and final time, Bowser must be hit twice to defeat him, allowing the player to earn the Grand Star.

Bowser's attacks are similar to the last battle. He begins by sending out three shockwaves the player has to jump over or avoid and Bowser now has a spin attack of his own that he uses when Mario is close by. After attacking him the first time, Bowser begins blasting fireballs around the planet followed by sending out more shockwaves and spins. Additionally, if the player counters Bowser's spin with their own, Bowser stops spinning momentarily.