Our skin epidermis is made up of a thin lining of epithelial cells that are shed occasionally by the skin to replace them with new cells. However, frequently instead of dropping off, these cells penetrate deep into the skin and cause sebaceous cysts. The sebaceous cysts are generally round, smooth, soft, and whitish-yellow. When an infected sebaceous cyst erupts, a foul-smelling, whitish-gray, semi-solid substance oozes out from it.