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A dark post is an unpublished social post that is promoted as an ad to a specific audience.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 though the term "dark post" makes this content seem mysterious, you should keep in mind that any user can now view all of your Facebook ads through the page transparency feature.