Historicity
Although mentioned in the Book of Daniel, the works of Flavius Josephus, and Jewish midrashic material, Darius the Mede is not known from any other primary historical sources. Neither the Babylonian nor the Persian histories record such a person. Herodotus, who wrote his Histories about 440 BCE, records that Babylon fell to the Persian army, under the control of King Cyrus, who had conquered the Median Empire as early as 550 BCE.
The conqueror of Babylon was Gobryas, governor of Gutium, a general of Cyrus the Great, king of Persia. A successor of Cyrus as king of Persia was known as Darius the Great. John J. Collins, who shares the Maccabean thesis view states that the author of Daniel inherited a schema of four kingdoms in which Media preceded Persia. He suggests that it was highly probable that Daniel created the figure "Darius the Mede" to fit this schema. However Darius the Mede is never described as king of the Medes, only as king of the Chaldeans (Dan 5:30-1, 9:1), and the Book of Daniel never mentions an independent Median kingdom, only the joint Medo-Persian kingdom (Dan 5:28; 6:8, 12, 15; 8:20).
Read more about this topic: Darius The Mede