Day 22 eggs with dark areas on shell?

[BarnyardChaos]

Some of the remaining eggs in my incubator are turning dark on the shells! It's day 22. Never seen this before. Is this a sign they're dead in the shell and about to explode? Or nothing to worry about?

 

[Tonyroo]

Candle carefully.

 

[BarnyardChaos]

I tried. I'm not very skilled at it in the late stages - I've candled lots of eggs, but have yet to identify an internal pip. Just can't see it. I candled all ten eggs - one definitely quit a week or more ago and it's out now. The other ten are all dark except for the air cells, and a small area underneath the air cell that's lighter, so I can tell the chicks haven't completely filled the shell (are they supposed to?). In a couple of them, the edge of the air cell on one side is uneven, not a perfect circle around the egg. Is that possibly pipping? I can't see any movement. Sorry I don't have pics - I can't get a good candling pic, and I've handled these poor eggs far too much already.