What is dark post on facebook?

Facebook dark posts are News Feed style ads that don’t publish to your Timeline or in your fans’ feeds organically. A dark post doesn’t have to just be a status update; currently, Facebook supports status updates, photos, videos, links or an Offer.

Why Use Facebook Dark Posts?
Generally speaking, dark posts are a good way to advertise without alienating or annoying your followers. More specifically, dark posts offer four major benefits:
You can get hyper-targeted. With the ability to create as many ads as you want without spamming your followers, you can create hyper-targeted ads for every segment.
You can do A/B testing. For example, you might change the image, headline, body copy or call-to-action button, or change several of these components for more variety. Use this to determine which variations have the highest click-through rates. The higher the CTR, the more money you’ll save on cost-per-click campaigns.
You guide your organic traffic better. Because you can see if certain images or headlines work better, A/B testing enables you to use paid social ads to guide your organic social strategy. Once you’ve determined which combinations perform the best, apply them to your organic posts for better reach and engagement.
You avoid ad-only streams. Ad-only Facebook streams are not only aggressive, but they tend to push away your audience. Facebook dark posts are niche and hyper-sensitive so you don’t blast the same CTAs to everyone through your feed.

How do you create an unpublished Page post?

An unpublished Page post is a Page post that will appear in News Feed but not on your Page. Unpublished Page posts are often used for creating News Feed ads that won't appear on your Page.

To create an unpublished Page post:

Go to Power Editor.
Click at the top of the page and select Page Posts from the menu.
Choose the Pages dropdown on the left side and select the Page you want to create an unpublished post for.
Click the blue Create Post button.
Choose the type of post you want to create and enter the info you want to include in your post.

select This post will only be used as an ad if you only want to use this post as an ad or select This post will be published on the Page if you want this post to appear on your Page.
Click Create Post. Keep in mind this post isn't published yet. It's saved in the list of your posts. If you publish the post on your Page, you'll have the option to only show the post to specific languages and locations.
Once you're ready to use the post, check the box next to it and select one of the options in the Actions dropdown.

Hope this helps