Zero Dark

1
A territory within the Moon Cell, yet separate from the Moon Cell itself.
The Moon Cell entirely blocked off the photon network leading to the Zero Dark territory, in order to sequester the Ark of the Stars, which is effectively an external memory vessel that the Umbral Star fired into the Moon Cell.
Even though the Moon Cell also has the Imaginary Number Space where it seals in malicious data, that is still considered part of the Moon Cell itself, as it is merely the Far Side of the Moon.
The Zero Dark, on the other hand, is a region that the Moon Cell treats as if it does not exist.
No physical connections remain to the Zero Dark, so it cannot become corrupted, but neither can it be purged.
It exists inside of the Moon Cell as a completely null space.
Its name, the “Zero Dark,” comes from the fact that it is a state of darkness that exists before dawn of civilization, before life and creation.
Ultimately, it is the territory of the Umbral Star within the Moon Cell itself.
Unfortunately for the Moon Cell, only its physical connection with the Zero Dark has been severed, so it is actually possible to enter via the right means. The only reason why the forces of the Umbral Star did not leave the Zero Dark sooner is because Titan Altera was “asleep” at the time.
The game establishes that Titan Altera cannot leave the cavern within the Zero Dark.
It certainly looks that way, because her body is shoved into such a tiny space. The reality, however, is not so clear-cut, and Titan Altera is in fact able to leave of her own volition from the moment she “awakens.”
However, if she were to escape outside, her ego would disappear, and she would turn into a mindless White Titan who exists solely to carry out Velber’s orders. She is afraid of becoming a mindless soldier, and so she chooses to remain inside of her own free will.
On the other hand, there is no danger of becoming an Anti-Cell when acting through Hero Altera, thus avoiding being attacked and having her consciousness erased by the Moon Cell.
It’s possible that the only reason why Altera conquers SE.RA.PH with the Main Character at her side is because she wants to see the continuation of a certain dream.

Translator's Notes
  1. ^ 未明領域 literally translates to “uncharted area.”
Fate/EXTELLA material: Encyclopedia of Fate EXTELLA