2-3 inch long, 1/2 inch wide vertical crevice/indentation just off to the right on top of my skull. only just noticed it. what could it be?

Probably lifetime: Sounds like an incomplete closure of a suture in your skull. Not unusual. A a kid these thing close up and the cartilaginous borders harden--but no always. If you remember severe head trauma, it could be a cracked skull with irregular healing or mild depression. Can't think of anything currently serious but a cracked depressed skull fracture could have been.