DIAGNOSIS That is the physical junction where the two glass panels meet. It is a normal characteristic of this monitor's manufacturing, not a defect. It may be more visible on solid color backgrounds.
FIX There is no fix. This is a design trait of the dual-panel assembly used to achieve the 57-inch size and extreme curve. Your only recourse is to adjust viewing distance or background colors to minimize its noticeability.
IF THAT DIDN'T WORK If the line is severe, discolored, or has dead pixels along it, then it is a defect. Contact Samsung support for a warranty assessment. Provide a clear photo.
HEADS UP Do not attempt to press on or clean the seam aggressively. You can damage the panel seal. Use a soft, dry microfiber cloth for cleaning only.