Messenger is still very popular among instant messaging app users, despite Facebook's own WhatsApp taking a huge piece of the cake in regards to popularity this past couple of years. The reason behind the former holding its pace is that it has tighter integration with Facebook and benefits a ton from that.
All of that aside, there is finally a long-long-asked feature inside Messenger - dark mode (or black mode, which is even better.) Previously, it was possible to enable it with root access and such, until they finally decided to hide an easter egg in the app lie Snapchat did and here is how it works. Simply, all you have to do is send a crescent moon emoji (🌙 - this one in particular) to anyone in a chat and you will get a pop-up saying that you have found dark mode, so you can go ahead and enable it from the settings. The funny thing here is that immediately after sending the emoji, moons start raining, or bananas, I would say. But back on topic, by performing this very same action, I was able to get dark mode to work, no issues so far, but in case it does not work for you, you may need to long-press on the moon emoji and Bob's your uncle.
Dark mode combined with Facebook Messenger's recently rolled out radical UI redesign makes up for a great chatting experience overall. Keeping in mind that it has gotten simpler and prettier, kind of, makes up for the huge data breach scandal we found out about last year.
I sincerely do hope a similar black mode gets implemented in all apps because I am a fan, but it looks like that is going to stay a dream for quite a while. Let's wait and see when Facebook makes it available to everyone.
Source: XDA Developers