The Upper Domain, or Higher Domain, transcends humanity's Lower Domain. It is the domain where U-DO exists. The two domains possess an intimate link as if they are melded together without being detached worlds, yet, despite that, they are existences that whiff past each other and cannot be seen. The given names "Upper" and "Lower" for the domains are nothing more than relative names. It is like how the third dimension is to the second, or how the fourth dimension is to the third. Each possess an individual time axis and space axis, but as a whole, they are elements that form one universe. In addition, it is unknown even whether or not the lower domain's concept of time exists in the upper domain in the first place. But regardless, although the existing role and function differ in each due to their difference in structure, the lower domain is indispensable for the upper, and the upper domain is necessary for the lower. This is because they form a single universe with both parts included. Therefore, the dysfunction and collapse of one will exert an effect on the other as well.
In the lower domain, where mankind exists, two mutually complementary domains coexist: the real number domain and imaginary number domain. A dual-layer structure similar to this also exists in the upper domain. However, the lower domain existence of mankind is unaware of what kind of state the dual-layer structure of the upper domain is in. This is because it is impossible to perceive and understand the structure of a higher-level domain from a lower-level one.
Ones other than U-DO also exist in the upper domain though it is the only one that accessed the lower domain is U-DO. Because the lower domain is not able to perceive the upper domain without there being access from the upper domain, as a result, the lower domain can perceive only U-DO's existence.
When access to the upper domain from the lower domain was successful, an overwhelming amount of energy begins to flow into the lower from the upper, originating from U-DO's wave. Due to the use of that energy, it became possible for phenomena that were thought to be impossible in the lower domain to occur. This is something overwhelming, as if it is the "manifestation of the power of God." For that reason, mankind sought access to the upper domain. They were willing to attempt it even in exchange for the risk of completely changing the structure of the universe due to a matter shift. Additionally, there also exist people who can directly draw out this power even without assistance from an artificial system. It refers to people, like Shion Uzuki's mother, who could directly access U-DO.