Dark air cell?

FrostyPaws

Hatching
7 Years
Sep 1, 2012
7
0
7
Interior Alaska
I have a batch of duck eggs that just went into lockdown last night and one of them has a dark air cell. It happened gradually over the last 2-3 days and now I can clearly see the shape of the cell on the outside of the egg without candling. The duck is still alive (I can feel him moving). Anyone know what is causing this and if it is a problem? I've been searching but I can't find any info about it.
 
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