What is it when you have brown shading between your boobs buts it's not dirt wont go away and comes back even when getting in the shower its been there for 3 + years and i also have black spots on my butt that look like acne but isnt and its spreadin?

One cause of brownish pigmentation between the breasts is insulin resistance causing acanthosis nigricans. It's a sign that your body isn't using all the insulin your pancreas secretes. If this pertains to you, avoid concentrated sweets like candy,pastry,syrups,sodas,etc & reduce starches like bread,pasta,rice. Also exercise daily. The black spots could be related. A dermatologist can advise you.