The Great Fusion
The Great Fusion was an incident in which many (the number is still unclear) dimensions suddenly connected with each other,
via semi-stable Wormholes.
It is believed that a race from another dimension called the
Tesla were the cause of this incident.
As they have supposedly, somehow, exceeded their universe' limit of space curvature, causing their dimension to 'break apart' so to speak.
This event caused it terms the great fusion.
Since the great fusion, laws of science of all the connected dimensions have merged.
However the full grasp of it is still unclear.
To identify their own dimension, Humans have called their dimension
Terra.
There are 2 dimensions connected to Terra which are publicly known:
Malus, home of the
Kyuuketsuki,
and
Aedes, home of the
Alchemists.
Both of which have very good relations to Terra.
One Dimension was discovered by the
APAR of Terra, which is called
Tabula Rasa.
However, there were only 2 exploration attempts. The second involved
Nova Okita, but at the beginning of the story Nova has no memories of this mission.
Because of the strange and dangerous nature of this dimension, it has been hidden from the Earth population by the government.