How do you create a static column header centered above your dynamic columns? One way to try achieving this is to place a textbox above your dynamic columns. One thing is for sure, the textbox will never be on the perfect centered location and what if the number of dynamic columns grow or shrink? 
 
Thing you need to do is to create a static column group. You do this by adding a new column group to the matrix and give it a static expression, for example: =”static”
Now make it the top group by clicking Up for the static column group on the Groups tab of the matrix’s properties. You can also achieve this by just dragging the column group up in the layout view.