Suggest Cause And Remedy For Large Bruised Area On The Back

I have what appears to be a large bruised area on my back. It has been there since the age of 14. I'm now 28. It has never hurt. The bruised area seems slightly depressed or lower than the normal skin areas on my back. Over time the color of the bruise has faded to a light tan/purple but started out looking like a full-fledged bruise. Do you know what this is? No doctor has been able to give me an answer.