Cora is a nearly indestructible, somewhat fluid material that can be found on the outside of beyond bubbles. Cora is the material of which these bubbles are constructed of, so therefore, The Guardian Of The 5th Wall can create it easily.

Omnipotence^1 entities or beyond can pass, or simply touch the material, so normal people can't look other realities. This can be created or destroyed by any omnipotent^G64 entity or above, but entities like The Aspect Lords or The True God have no reason of creating it.