Explain why in the XI – IX centuries. is the history of Greece called the Dark Ages?

Most likely they mean centuries BC. That is: XI-IX centuries BC
At the beginning of this interval, there was a Dorian invasion from the north of wild tribes who destroyed the large cities of the Mycenaean culture. There was also a strong decline in writing and culture. In fact, in the 9th century BC. the development of Greece began anew, almost from scratch. That is why the name “Dark Ages” was assigned. This time is also called the Homeric era, since the “Iliad” and “Odyssey” are in fact the only sources about this time.