The "/phantoms" page:

Show a list of all "phantom" artifacts that are not marked as "private".

A "phantom" artifact is an artifact whose hash named appears in some
artifact but whose content is unknown.  For example, if a manifest
references a particular SHA3 hash of a file, but that SHA3 hash is
not on the shunning list and is not in the database, then the file
is a phantom.  We know it exists, but we do not know its content.

Whenever a sync occurs, both each party looks at its phantom list
and for every phantom that is not also marked private, it asks the
other party to send it the content.  This mechanism helps keep all
repositories synced up.

This page is similar to the /bloblist page in that it lists artifacts.
But this page is a special case in that it only shows phantoms that
are not private.  In other words, this page shows all phantoms that
generate extra network traffic on every sync request.