It depends on two things:
1. How important is it to the author to have either the MC or the reader know what really happened? Sometimes you need to tell them everything, sometimes only the reader needs to know (for example, the gods of the Diskworld are playing a role-playing game with dice and everrything in "The Light Fantastic" and "The Colour of Magic" - but Rincewind, Twoflower and The Luggage never see this, only the reader does), very VERY rarely the MC figures it out and refuses to share with anyone (including the reader - I have seen this attempted three or four times and see it work exactly once, so would not suggest it, but it is an option; wish I could recall an example).
2. Do you plan sequels/spin-offs/follow-ups? If so you WANT to leave some mysteries for later stories to explore (stories that may have entirely different MCs - and which may even show the "answers" revealed in previous installments were incorrect or flat-out WRONG, as in the "Fate" anime series)