Petrach is often viewed as coming up with the idea of a "dark age". However he likely did not use the phrase itself, instead he created the concept that the time between Antiquity and the Renaissance that started in the 14th century (Petrach was writing in the 14th century). The reason why is pretty simple, Antiquity was seen as the model and the Middle Ages were seen as a decline from that ideal. Much of casting the Middle Ages as the "dark ages" has to do with framing their own time period as a new age, or a "lighter" age.

If you are looking for a more in depth discussion of Petrach I would recommend the article "Petrach's Conceptions of the Dark Ages" by Theodore E. Mommsen (I got it from JSTOR, so if you have access it should be easy to find). Hope this helps!