A hole-shaped hole.

Greta Christina hits the nail right into the wood again, with her piece "There has to be somethng more": atheism and yearning.

Toward the end, she takes her argument to a conclusion that I found particularly striking (maybe it works better in context):

We don't have a God-shaped hole in our hearts. We have a hole-shaped hole in our hearts.

I think there's something profound in that little epigram.