A new feature in Facebook's Messenger app - Dark Mode - has been in the works for some time now. A Redditor has recently noticed that it can be activated by sending a specific emoji. The new Mode may particularly benefit those with light sensitivities or whose devices have OLED screens.
https://www.notebookcheck.net/Facebook-Messenger-s-Dark-Mode-is-now-available-for-activation.412693.0.html