It's a 'fairy egg'.
That egg would not be viable for hatching.
There might not even be a yolk in there.
Open it up and see whats there, post another pic if you want.
Fairy, fart, wind, rooster eggs are usually from a tiny piece of tissue breaking loose from the reproductive tract, or an immature ova(yolk) and the body forms an egg around it. Color can be darker than 'normal' as the pigment coating released has to cover a much smaller area so is thicker. Can happen with any age layer, but more common with new or older layers.
