According to Facebook, “After a month-long Easter Egg activation with countless crescent moon emojis, we are excited to announce that everyone around the world will be able to activate dark mode in Messenger with a simple toggle switch from settings. The toggle is rolling out globally today.”
If you’d like to enable the feature, all you’d have to do is tap your profile photo in the Messenger app to access the app’s settings, and then click the toggle on dark mode to switch the app from its current light theme to a dark theme. While this might seem to be purely an aesthetic choice, dark mode does help reduce glare when using the app in low light settings.
In some cases it can even help to save battery life, which Google themselves had previously acknowledged in their own testing. The feature should already be live so make sure you have the latest version of the app and you should be good to go.