Translation:The data shows that it is a good theory.
That's a myth. Both usages are standard (and are accepted here).
Thanks, Giorgio. It depends on your generation. Many people of a certain age who were educated in the sciences see "data" as the plural of "datum". However, common use is moving away from that. So I would say that we are in a transition period in which "data" is treated as both singular and plural. Both forms will be accepted, and we will not engage in a dispute about which one is correct.