What Causes A Small Round Dark Spot On The Glans Of The Penis?

Several months ago I noticed a small round dark spot on the glans of my penis--about the size of a shirt button. It is a little deeper pink than the rest of the glans. It causes no discomfort and has changed in all that time. Is it normal? Do I need to get it checked? I've been in a monogamous relationship for 20 years. I'm 40 years old.