God intends to bring about a permanent physical separation of light from darkness. Light weight and light bearing are synonyms, like dark and heavy are. In general heavier, denser materials are darker.
Physical Separation
Why Have You Forsaken Me?
Light and darkness have to be separated because the manifestation of free will is only possible by combining them.
From the Sixth Hour to the Ninth Hour
Another picture that we have which is part of the symbolism of Christ on the cross is that darkness came upon the world in the middle of the day. Things on Earth are patterned after things in the heavens.
Darkness on the face of the Deep
The nature of free will is such that an individual can choose light or darkness. How can there be free will unless a choice is available? The Lord had to create a universe in which darkness exists.
Light = Good; Darkness = Evil
The parallels between light and good vs. darkness and evil are stated throughout scripture. Isaiah laments people who try to make you believe that good is evil and evil is good, and who put darkness for light.
The Broad Narrative of Scripture
The Bible is consistent throughout because it combines spiritual concepts, like good and evil, with their physical expression in space and time as light and darkness. It describes their permanent physical separation.
December 8th
Christ's sacrificial death on the cross and the darkness immediately after it picture creation in the moments before God said "let there be light" before nucleosynthesis, the fabled "Big Bang."