Just a funny occurance with several funny names; Fairy eggs, wind eggs, fart eggs. They may or may not have a yolk. Safe to eat. I've only had 2 but I save them to dry up and be decorative.
Do you have pullets just starting to lay? Sometimes this happens with beginners. May not happen again. Is kind of a practice egg as the egg machinery sputters to a start. Can also happen at the other end of a hen's laying cycle, I believe.
I got one of those with a quail egg, so it was the size of a small marble. Cutest thing ever but there wasn't anything in it. They are infrequent but nothing to worry about.
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.
I think it's explained in this excellent video, which is worth watching regardless: