The Void

[Zigûr]

It might also be worth pointing out that it's unclear which "Void" Morgoth was cast into at the end of the First Age.

We read that he was then thrust out into the Void. That should mean that he was put outside Time and Space, outside Eä altogether; but if that were so this would imply a direct intervention of Eru (with or without supplication of the Valar). It may however refer inaccurately to the extrusion or flight of his spirit from Arda. (Myths Transformed)

Professor Tolkien doesn't actually resolve this dichotomy within the text.

Ainuldinalë also implies that the Void has spatial dimensions:

it seemed to him that Ilúvatar took no thought for the Void, and he was impatient of its emptiness

This could be metaphorical, however, and it is possible that what Melkor saw as "emptiness" was in fact pure "nothingness"; in this sense Melkor misunderstands what the Void is, which might correlate with why he sought the Flame Imperishable there. Perhaps Melkor's fëa could "explore" the Void in the same way that the mind can think about nothingness - his fëa went somewhere and searched, but in a spiritual sense rather than a physical one.

I really think that to a degree the Void is beyond human comprehension...