Source : Webster's Revised Unabridged Dictionary (1913)

Somberness \Som"ber*ness\, Sombreness \Som"bre*ness\, n.
   The quality or state of being somber; gloominess.

Source : WordNet®

sombreness
     n : a state of partial or total darkness; "he struck a match to
         dispell the gloom" [syn: {gloom}, {somberness}]