There is a raised bag at the back of the head, the common causes are the following: First, the most common is sebaceous cysts. The so-called sebaceous cyst is a normal secretion of sweat by the sebaceous glands, but the surrounding ducts are blocked by sweat or other tissues, resulting in the normal discharge of sweat and long-term accumulation, causing the sebaceous cyst to gradually increase in size. Secondly, the common cause is lymph nodes. Infection of the surrounding tissues can cause enlargement of lymph nodes, which are generally mobile and have a smooth surface.