Have a bump in the top middle of my back that feels like a big marble & is visible, has been there for yrs, only hurts when pressed. what could it be?

Not uncommon: Sounds like a sebaceous cyst. It forms when the hole that sebum comes through gets covered over yet the gland keeps making sebum. Fast forward years and you have this lump. Sometimes they burst and they have the grossest stuff in it. I opened one up in clinic and the inside hit the ceiling. They recurr if drained so taking them off is the only absolute cute. Hot compress can help if it's sore.

Sebaceous cyst?: Am not an md but have one of these. Have ur doc check it to make sure it's not something else. If it is sebaceous, i wet and rub a styptic pencil over the surface then let it dry, over several nights before bed. After a while, it softens and u can squeeze the goo from it (like a big pimple). U can also have it surgically removed but cysts often come back anyway.