From the screenshot it appears your grid width is > than sum total of visible columns ( and there is no horizontal scrolling)
to remove empty space,
1) either you have to shrink the grid's width to sum total of column widths. use flexWidth : true in this case.
2) or expand columns to fit properly in grid. use scrollModel: { autoFit: true, other options... }
There is also a 3rd option scrollModel: { lastColumn: 'auto', ... } which works well in virtual mode and when there is horizontal scrolling.