I have a semi large hard lump on the back of my skull, it's painful but it has no head and no sign of opening up. what would you think it is?

How long do u have: There are few things of the consistancy you are saying can happen. Few of the common ones are sebacious cyst - usually has a punctum, dermoid cyst which is possible in your case which is a firm to hard lump usually , fibromas, lipomas which are usually soft but if they have some fibrous tissuethey can be firm , neurofibromas arising from nerves , hamartomas-of mixed issue content, teratomas.

Lump: please see your GP for exam. most likely a sebaceous cyst but unless seen, cannot be commented on.