This can be tricky.. Let me try to give an explanation.
In your first example, the correct response is "he left at 6:00." Simple past. Tells when action happened.
If it happened at an unspecified time, you can use either "left" or "has left. "He has left the terminal" or "He left the terminal." "He has left" stresses that it happened in the recent past.
In the second example, you write, "the galaxy was formed ..." Or "the galaxy formed" . Both simple past. And you can't use "has been formed" because it happened long ago.