Facebook is planning to roll out the dark mode feature in its messenger. It will take time but this feature is currently hidden inside the app. All you need to do is to trigger through an emoji and you are all set to use the dark mode before it is official.
All you need to do is send a crescent moon emoji message to anyone in your friend list. After you have sent the emoji, the chat screen will see a rain of crescent moons. Just wait for a moment and a notification will pop up saying you have found dark mode. Just tap turn on and you are all set to use the hidden dark mode.