Dark traffic is non trackable, non, traceable direct traffic that that doesn’t provide referral information. In other words the traffic is directly to your site or in this case your links. Dark traffic is simply put traffic that is coming in from the dark.
So in other words, we don’t know where it came from. We don’t know the source.
The user has directly added your link address in the browser but there is no way we can know where the user got this information from. Another reason is, because the user came from a closed platform or a mobile channel where we cannot see it, like for example SMS, twitter cards; so it classifies this as direct traffic instead.