Darkness can result due to a problem related increased weight known as Acanthosis Nigricans and the skin typically looks dirty and dark . This seems to be the likely Skin problem in your though only physical consultation and examination can confirm it.
Best Regards,
Dr. Ritupurna Dash MD
Dermatologist & Aesthetic Surgeon
DASHDermatology
Next Steps
It indicates insulin resistance in the body, so without further delay do consult a dermatologist.