Visible seam on the mesh I have started to model a torso. I was happy with it, until I discovered the seam in the mid of the mesh. I have not used the mirror modifier at all, what else could it be?
It seems that half of the normals in your model are inverted (the normals of some faces are facing inward and some outward). This might be causing display problems.

Go to Edit Mode, select all faces and use CTRL + N to recalculate normals.

If recalculating the normals doesn't help, display face normals as lines, select only the faces with normals facing inward, and flip normals on those faces.