Obviously, you'll want to make sure that your Messenger app is updated before you try this, lest you end up sending crescent moons with no reward.
Facebook has chosen to add this feature in a pretty unconventional way, too: To get the dark mode option in Messenger settings, you need to send someone the "crescent moon" emoji. If trouble persists when trying to turn on "Dark Mode", users should try updating the app. It's not clear why it didn't come with an obvious button or toggle in the settings menu that lets users quickly enable it - but it's not to hard to unlock it yourself. This will let you know that you have successfully unlocked the feature and it can be toggled On or Off within the app's settings menu.
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Users are then prompted to turn the feature on in Settings.
Reddit user FusionxGamerz spotted the hidden method of enabling what I like to refer to as "Emo Mode", sharing it with users on the forum. Tap it, and it should bring you to your Messenger profile. "Dark mode cuts down the glare from your phone for use in low light situations, so you can use the Messenger features you love no matter when or where you are".