Hard bumps on the inside of the skin may be caused by dermatofibroma, epidermal cysts and acne. 1. Dermatofibroma: Dermatofibroma may be caused by trauma, viral infection and insect bites, etc., and the patient’s skin will appear firm nodules, the color of which varies from yellowish brown to black. 2. Epidermal cysts: embryonic ectopic, hair follicle structure is destroyed and other factors may trigger epidermal cysts, patients will appear in the skin of the dome-shaped swelling, cysts hard texture, some patients will appear inside the skin of the hard pimples and black. 3. Acne: Acne may be triggered by endocrine, genetic, microbial infection and other factors, the patient’s skin will appear blackheads, pimples, nodules, pustules and other symptoms. There are many reasons for the blackening of hard bumps on the inside of the skin, and it is recommended that patients seek medical examination to clarify the cause of the disease.