Skin darken patches at the back, Any advice for cause or treatment ? (images)

[minkian]

I have problem with my skin colour at my back which is darkening. Probably I have this dark patches for about 15 years. however recently I feel they are growing and getting worse.  I have done many type of blood test during this year, not for my skin problem, however all the result showed my health are in good condition.
I guess about liver mulfunction or insulin resistant or acanthosis nigricans based on my issues.
I'm 30 years old male, height 172cm, Weight 84 Kg (I have mainly belly fat 5-6kg )
these darken area don't have any symptoms like itchiness but I am worried that they could be cause of one or more organs malfunction or skin cancer.

Images:
https://postimg.org/image/km8wcnzlx
https://postimg.org/image/rqqplp6v9

I'll be appreciate for your advice.