In this verse we read that Darius the Mede became king. The problem here is that we do not read anywhere outside of the Bible that a certain Darius became king at that time. The sources outside the Bible only speak of Cyrus or Kūruš. He would be the successor of Belshazzar.
Yet the problem is not as big as it may seem at first glance. There is a good solution for this. It is clear that the Medes and Persians formed a coalition against the Babylonians. They fought and conquered together. Cyrus, who was a Persian, was their leader.
Everything indicates that for the first few years after the fall of the Babylonian Empire, the Persians and the Medes ruled the great Empire together. The Medes also receive a reward for their part in the victory. One of their leaders shared the kingship with Cyrus. The striking wording in Daniel 5:31 that Darius received the kingship may give an indication of this. Especially when we also pay attention to Daniel 6:28: So this Daniel prospered during the reign of Darius and the reign of Cyrus the Persian.
The word and
(between the names of Darius and Cyrus) does not necessarily imply that Darius and Cyrus were kings one after the other. It can also mean that they shared the kingship together for a certain period of time.
One thing is clear: Darius really did become king. And the Lord did fulfill his earlier prophecy that Babylon would fall: Isaiah 21:9; Isaiah 44:24–28; Isaiah 45:1–7; Jeremiah 25:12.
31 And Darius the Mede received the kingdom, being about sixty-two years old.