Dark Post

A dark post is an unpublished social post that is promoted as an ad to a specific audience. It's a post that can't be found organically and doesn't appear in your brand's Timeline. Dark posts can help with split testing and prevent your Timeline from being flooded with promotional posts.

Even if the term "dark post" makes this content seem mysterious, you should bear in mind that any user can now see all your Facebook ads via the page transparency function.