What Causes Dark Skin On Neck And Under Arms?

Hi I'm 20 years of age and my current weight is 165. I have no medical conditions, thank god. I do have a consurne about my skin. Around my neck and underarms it's very dark almost like if it's to be dirty and I don't shower. But I shower everyday and scrub my body as well. I dont understand why it looks that way? Can I bleach my body