Yes, hens lay unfertilized eggs. They go through a process called ovulation according to light patterns in which they do not need any rooster (male fowl) to lay eggs. However, hens only lay a fertilized egg when mated with a suitable rooster. An adult hen ovulates soon after laying the previous egg and it takes around 24 to 26 hours for an egg to fully form.
Hens have a life span of 6 to 10 years during which they produce eggs for 3 to 4 years only once they are 18 to 22 weeks old. For commercial egg production, they are kept only for 2 to 3 years as after this period their egg-laying capacity decreases. Moreover, egg-laying breeds(Layers) can not be used for meat production in the poultry business as their meat yield is very low.
You may not have eaten fertilized eggs in your lifetime because nearly all eggs sold in the market are unfertilized. One can not tell the difference between a fertilized or an unfertilized by only looking at the outer covering (shell) of the egg. However, the difference lies in the involvement of roosters in the egg production.
When hens are mated with roosters, fertilization takes place, and under the right conditions (incubation) it can produce chicks. Moreover, even if fertilized eggs are sold in the market, there is very little chance of developing an embryo as it requires optimum conditions for growth.
Many people who notice a blood spot inside an egg, initially take it as a sign of a developing embryo. But, this is not true. Often due to rupture of blood vessels in the hen’s reproductive system, blood spots may be seen in unfertilized eggs. In fertilized eggs, you can notice veins development on the very fourth day of incubation.