Epidermoid cysts occur when the top layers of skin fold in on themselves, blocking the dead skin cells from shedding. When these dead cells move deeper into the skin and multiply, they sometimes form a keratin-filled sac.
Anyone can develop a cyst, but certain risk factors make it more likely. If you sustain an injury to the skin that damages a hair follicle, you’ve been through puberty, or you have a history of acne, you’re more likely to get skin cysts.