Helpme undestand MDX exists function
If you are browsing in Excel and want to be able to see dimension attribute members that do not have corresponding rows in a fact table (measure returns nothing), there is a pivot table setting you can use. 

Access the PivotTable Options dialog box by right clicking on the pivot table and choosing PivotTable Options. On the Display tab, check the box next to Show items with no data on rows. This will show all values of whatever hierarchy is in your rows regardless of if the measure in the Values section of your pivot table returns a non-blank result.

 

You can do the same thing for columns by checking the item underneath the one indicated in the image. This option must be set for each pivot table and will apply to all fields in the rows (or columns) when you check the box.