What does the Dark and Darkness represent in Emily Dickinson's "We Grow Accustomed To the Dark?"

In the first two stanzas, one could say that Dickinson is
talking about literal darkness, for which she uses the word "Dark." The Neighbor holds
up the lamp to say goodbye and then leaves, taking the light. It takes a few moments for
your eyes to adjust to the darkness. At a certain point, your eyes do adjust and you are
able to move forward, although you may be apprehensive and your steps might be
uncertain. 

In the remaining stanzas, Dickinson shifts from
the word "Dark" to "Darkness" and this is more figurative, metaphorical. This
metaphorical darkness, she describes as "Those Evenings of the Brain - / When not a Moon
disclose a sign." This is when we are mentally in the dark: a result of being
spiritually lost, depressed, unable to find an answer or solution to a problem,
uncertain, etc. Just as our eyes can eventually adjust to the "Dark," our brains can
adjust to the problems of mental darkness. It takes "Bravery" and one might get hurt in
braving his/her own mental darkness "And sometimes hit a tree" (here, Dickinson shifts
back to walking in actual darkness, once again as a metaphor for mental
darkness/depression.) 

In the end, "the Darkness alters"
(things get better), or we become better at dealing with
things: 

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Something in the
Sight 

Adjusts itself to Midnight
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