I've got dark spots on my penis, I think they've always been there, but I was just wondering what causes them?
The medical term for dark spots like these is 'lentigenes' (singular: lentigo). They are caused by proliferation (an unusual rate of production) of melanocytes (the cells that produce melanin pigment), but in two dimensions only. (If they proliferate in three dimensions the result is a mole.) The cause of the proliferation is not known but seems to be something temporary and local in the foetus. They are not cancerous or dangerous.