Facebook Messenger application currently testing Dark mode in some countries. However, a little trick can help you activate it very soon. This trick can be applied to both iOS and Android. The steps are as follows:
Step 1: Install the latest Facebook Messenger version. If your device has not been updated, please update.
Step 2: Go to Facebook Messenger, open a chat window with anyone. If you are afraid to bother someone, you can open a chat window with yourself (You can do it).
Step 3: Click on the emotion icon. Go to the icon of the moon tongue as shown below (in the ANIMALS AND NATURAL set) and send it.
Step 4: The message “You Found Dark Mode” appears. Click on Try it in Settings.
Step 5: Turn on Dark Mode.
Step 6: You get instant Dark mode for Messenger.