Facebook introduces a dark mode in its Messenger. You can only enable it by sending the crescent moon emoji in an existing chat thread or by sending this emoji in a new message. It seems like Facebook has taken this step to encourage users to chat more instead of simply showing the enable option in the settings. The decision of introducing dark mode by sending moon in a chat seems to have come from the heavy competition Facebook is facing with TikTok.
The moon is a limited-time only experience, says Facebook. It means the crescent moon trigger will be replaced soon with the feature simply accessible via your Messenger settings.