What is a scar under the skin called? how does scar tissue form if there was never an open wound?

Not the scar: The scar is usually not the problem. Early after the injury, the decreased movement is usually due to swelling and soft tissue injury. After healing, poor movement may be due injury to the nerve or muscles responsible for lifting the eyebrow. This will likely get better with time but will likely have reduced movement (compared to the other side) permanently.

Scar fibrosis: Even without an entry wound, an injury to the soft tissues (like a severe bruise) or bleeding within the fat or muscle can cause damage leading to permanent scarring. This scarring is firm and swollen at first, and can restrict movement. You could also have had injury to nerves (sensory or motor) that could lead to temporary or permanent damage to these functions. Time softens things significantly.