A 'void', or 'Vacuity' denotes a person's state before regeneration. From Arcana Coelestia 7: "The first state is the state which precedes, both the state existing from earliest childhood onwards and that existing immediately before regeneration; and it is called a void, emptiness, and thick darkness.

In Genesis 1:2, this signifies lacking good, as with man's state before regeneration. (Arcana Coelestia 17)

In Psalm 89:39, this signifies that the Jewish nation had destroyed its conjunction with the Lord and the church. (The Inner Meaning of the Prophets and Psalms 339)

The "midst" of something in the Bible represents the thing that is most central and most important to the spiritual state being described, the motivation that drives everything else. In general this will be something we love or feel, because at the core of things we are what we love; our loves define us.

(References: Apocalypse Revealed 44, 90, 268; Arcana Coelestia 2252, 2940, 10153, 10365, 10557, 10635, 10641)