Dark brown eyes are incredibly common among humans because of how gene inheritance works. There are two alleles that can be passed down; these are how the genes are expressed on the physical body. One is for brown eyes (which is a dominant color), and one is for blue eyes (which is recessive, meaning it only shows when the person has two alleles for this gene). The one for blue eyes can be mutated to give green eyes, but for the purposes of this answer, it will only correspond to blue eyes.
Brown eyes will be represented by “B”, while blue eyes will be represented by ‘b”. So, if someone has the alleles “BB” and marries someone with the alleles “Bb”, both people will most likely have brown eyes. All their children will also have brown eyes.