Hi Methee,
The universe has a "Limit the size of the result to" setting, it may be too low. You may need to tweak the connection in Data Federation tool and the APS hosting the DF service. Try creating a new Webi report with same objects and see if that works, it may provide more insight on the issue.
I can tell you that 7 million rows will not be easily indexed in Explorer, I'm going to say it will not be possible depending on how many facets and measures you select. You are way over the limit for non-acelerated data source in my opinion.
George