A dark archive is an archive that isn't available to the public, generally to preserve documents over longer periods of time for future generations [0].

In this case, documents are only made public/open-access with a certain trigger, e.g. when a document becomes unavailable online. This makes it immensely useful to find articles that almost no libraries pay for anymore.

[0]: https://dictionary.archivists.org/entry/dark-archives.html