If the streak runs from the first frame of the film to the last:(posted 8642 days ago)on 35 mm, it might be some dirt or debris lodged in the felt of the canister,
35mm or other formats, dirt or a nick on a roller in the camera,
if it's top to bottom on 35mm, or across the narrow size of the film, and only appears in one frame, it might be a defect in the film, a light leak, something abrasive on the dark slide of a medium format film holder, etc.
Tell us the format, kind of camera, etc. to help narrow it down.
Another possibility is static electricity, though I wouldn't expect it based on your description.
Was it every frame on the roll, just one or two?