Dark Meridian

In the Universe of Kingdom Hearts, also called the World, there are many worlds that fall into certain realms. All of these realms are on their own plane of existence.

According to the story told by Kairi's Grandma in the Hollow Bastion's Library, long ago, there was one single world that was constantly bathed in the warmth of Light. When people began to fight over the Light within their world, Darkness appeared in their hearts, and the world was consumed. However, fragments of light remained within the hearts of children. From this light, children were able to rebuild the lost world. However, darkness still existed, which caused the worlds to be separate from each other, divided, waiting for the true light to return.

It has been revealed the fight was between the many Keyblade holders over Kingdom Hearts, and eventually led to the Keyblade War.

The worlds were surrounded by barriers, to prevent outside interference, making it impossible for different worlds to have contact. The arrival of the Heartless caused the barriers between the worlds to crumble and disappear, and some worlds were even consumed by the Heartless. Gummi Blocks are actually pieces of these broken barriers.