I suffered head trauma and now my under eyes are darker - makeup is not hiding it at all. What do I do?

Blood was leaked and gravity placed it around the orbital region. The eyelid skin is quite thin and that is why you have discoloration. I would give it 6 months before you consider doing anything to correct this as it may resolve itself.

Best Wishes,

Gary Horndeski, M.D.

The injury has likely caused bruising to the lower eyelids. Even though the trauma was to your forehead, blood pools in the lower lids due to the effects of gravity.
This should fully resolved. Continue with your usual skin care regime.