After initially making 'Dark Mode' available via an Easter egg within Messenger - which required users to send a moon emoji in a chat - Facebook has now added the option to its regular Messenger settings.

As per 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."
To activate Dark Mode, tap your profile photo at the top left of the main Messenger screen to access your settings. You can then toggle on dark mode "to switch the chat aesthetic from white to black".

Apart from simply looking different, Dark Mode does have health benefits, in that it emits less blue light, which can disrupt sleep patterns when used at night.
It is an interesting switch-up too - even as an experiment, it's worth checking out Night Mode and seeing what you think.
Also, if you didn't get a chance to see the 'moon shower' animation as part of the initial Easter egg, Facebook says that you can still re-create it by sending a moon emoji in any chat.