The most likely cause is a shutter problem, specifically a sticking or unevenly traveling focal-plane shutter.

On this type of camera, at faster shutter speeds the shutter exposes the film with a moving slit rather than opening fully all at once. If that slit moves unevenly across the frame, one side can get less exposure, which shows up as a dark band along the edge.

Because the underexposure is along one side of the frame and you’re not using flash, this points much more toward shutter timing/travel issues than a lighting problem.

Typical fix: have the camera serviced with a CLA (clean, lubricate, adjust) by a technician familiar with mechanical film SLRs. That’s the standard remedy if the shutter is sticking or running unevenly. Be aware that on older cameras, repair may or may not be practical depending on parts availability and service cost, but the issue itself is a common shutter fault.