The Reverse World (Japanese: 反転世界 Reverse World) is another universe directly opposite the Pokémon world. It is where Giratina resides and it plays an important role in Giratina and the Sky Warrior.
It is depicted as an ultimate world where there is no solid ground where it might be expected and mirror images of scenery is in vertical symmetry with itself. According to Newton Graceland, it's a location of which is exactly opposite of the world of reality. The force of gravity in this world gets weaker or stronger depending on the subsequent areas or locations.
The Reverse World has a direct and symbiotic connection to the Pokémon world; even though these two worlds work side by side, they do not intersect at all. When there are imbalances in the real world, it takes in abnormal pollution from the Pokémon world, which takes the form of poisonous clouds and then corrects them to maintain the balance between time and space.
Many pillars and crystal shards freely float around the Reverse World, depicting things in the real world. If the shards are shattered or if any part of a pillar is destroyed, an explosion happens in the real world near the place or thing within the shard or whatever the pillar is currently mirroring. Giratina has the exclusive ability to stop a glacier from collapsing by repairing the damage it sustained in the Reverse World.
Light gets stranded in the Reverse World. There, he finds and fights Darkrai. Danny Phantom spots the brawl and a few moments later helps Light. Giratina sees the two and attacks. Eventually, Darkrai regains conciousness and Shalos appear. The new team joins forces to get to the Portal and leave the Reverse World.