A large, dark lump on the back can look alarming, especially when the surrounding skin is red, swollen, or irritated. While a photograph alone cannot confirm a diagnosis, a growth like this may be associated with an epidermoid cyst, an inflamed cyst, or another type of skin lesion. Epidermoid cysts commonly develop on the trunk and can gradually become larger, painful, or inflamed.An epidermoid cyst forms beneath the skin and may sometimes have a small opening in the center. Although these cysts are generally harmless, they can become inflamed, rupture, or become infected, causing tenderness, swelling, and redness. However, redness does not automatically mean an infection is present. Several different skin conditions can appear similar, so a healthcare professional needs to examine the area before determining the cause.Treatment depends on the size, symptoms, and condition of the lesion. A doctor or dermatologist may recommend incision and drainage when a cyst is painful or significantly swollen. In some situations, medication or an injection may help reduce inflammation. If appropriate, complete surgical removal can eliminate the cyst and lower the chance of it returning. When a cyst is severely inflamed, removal may sometimes be postponed until the inflammation improves.