What Causes Dark Spots On The Thigh?

my entire right side from waist down is covered and about three medium sized spots on my back are darker than my caramel complexion and i was wondering what could be the cause of this....im 24 yrs old and i've had this problem since i was about 12 or 13yrs old it jus grew with me and spread but i remember it started in the back side of my right knee