  With 60M articles in more than 300 language versions, Wikipedia is the
largest platform for open and freely accessible knowledge. While the available
content has been growing continuously at a rate of around 200K new articles
each month, very little attention has been paid to the accessibility of the
content. One crucial aspect of accessibility is the integration of hyperlinks
into the network so the articles are visible to readers navigating Wikipedia.
In order to understand this phenomenon, we conduct the first systematic study
of orphan articles, which are articles without any incoming links from other
Wikipedia articles, across 319 different language versions of Wikipedia. We
find that a surprisingly large extent of content, roughly 15\% (8.8M) of all
articles, is de facto invisible to readers navigating Wikipedia, and thus,
rightfully term orphan articles as the dark matter of Wikipedia. We also
provide causal evidence through a quasi-experiment that adding new incoming
links to orphans (de-orphanization) leads to a statistically significant
increase of their visibility in terms of the number of pageviews. We further
highlight the challenges faced by editors for de-orphanizing articles,
demonstrate the need to support them in addressing this issue, and provide
potential solutions for developing automated tools based on cross-lingual
approaches. Overall, our work not only unravels a key limitation in the link
structure of Wikipedia and quantitatively assesses its impact, but also
provides a new perspective on the challenges of maintenance associated with
content creation at scale in Wikipedia.
