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.
How to Fix It
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.