Hi Eduardo,
The report itself would provide the list.
I'm assuming you've been running reports based on dates (DATE, WEEK, MONTH, YEAR) so far, as they're the most common ones. In a daily report, for example, you get the date you're looking at as part of the report (as the first column).
In this case, your dimension (i.e., what your report gets aggregated by) would be the DOMAIN_NAME instead, thus being returned in the report as the first column.