Astral World is a world which exists in the alternative dimension parallel to the Barian World. It also where Astral was born from. It appears as a galaxy with blue and pink colors. It has quiet big fued against Barian World which seeks to destroy it.
While under exploration, Kazuma Tsukumo fell from a cliff in a snowy mountain landed in the Astral World, and gained the Emperor's Key from a sipirt which kind of look like Astral.
Sometimes later, Kazuma went on an exploration to find the door to parallel worlds with Dr. Faker and Byron Arclight. Dr. Faker betrayed him and Byron using them as sacrifices to open the door. The two was then sent to alternate dimensions - Kazuma ended up in the Astral World and Byron ended up in the Barian World.