I have dark brown spots underneath my armpit area, through out the sides of my ribs aswell. I am over weight, and im 28. What can it be?

Insulin resistance? Hard to say without seeing it, but i would suspect acanthosis nigricans, which is a darkening and thickening of the skin (which ends up looking "velvety") in the flexural creases, like the underarms, the neck, the popliteal fossa (inner elbow) and the groin. This is a sign of Insulin resistance, which is the mechanism by which overweight people eventually may develop diabetes.