The Black is a 'Between Dimension' of our Universe, encompassing the dark 'non-space' that exists on multiple planes of Bi-Quantum Reality. This 'non-space' is totally invisible to any manner of visual, aural, tactile, or indeed any other sensory perceptions.
The structure of The Black is largely unknown, however, a number of pioneering scientists specializing in the field of Bi-Quantum Reality related science have attempted to map at least a basic structure of it. Such scientists include Dr Bugs McBigglesworth, who has famously speculated that The Black exists and runs through our universe 'in a similar manner to the way in which veins run through the human body, albeit invisibly.'
The Black runs through multiple different planes of reality and universes, however, it is unknown whether it connects them in any way, as speculated by McBigglesworth.