The social media app has long been rumoured to be getting the tool which enables the screen to go black for night time - much like Twitter's version - and now some users have claimed that they have been able to turn their app into Dark Mode.
In order to activate the feature, users have to send the crescent moon emoji to themselves or a friend and then click on the emoji once it has sent.
According to Metro.co.uk, a pop-up message appears saying "you have found dark mode".
It's then able to be turned off in the settings.
Dark Mode is intended to make night time reading on the app easier.
It's known whether they are testing out the feature or whether it will become a permanent fixture on Messenger.
At the end of last year, Facebook was reported to be planning to integrate its messaging services on Instagram, WhatsApp and Facebook Messenger.
That was according to a report in the New York Times newspaper, which said that while all three brands will continue to be standalone apps, they will be integrated in order to ensure that messages can travel across the platforms.
It's been reported that the initiative is being driven by Facebook founder Mark Zuckerberg, who hopes to have completed the integration work by the end of the year.
Facebook, for its part, confirmed that it is busily working on some of its ambitious plans - although the company refused to reveal specific details about its efforts.
In a statement, the social networking platform explained: "We want to build the best messaging experiences we can; and people want messaging to be fast, simple, reliable and private.
"We're working on making more of our messaging products end-to-end encrypted and considering ways to make it easier to reach friends and family across networks."
Despite this, Facebook admitted it has already staged a lot of "discussion and debate" about how the proposed new system ought to be implemented.