Halliday & Matthiessen (2014: 538-9):
A fact clause serving as Head in a nominal group without a fact noun can be related to the first class of fact noun – that of ‘cases’, since such a fact clause is always agnate with an expanded nominal group with fact as Head. Whether the nominal group has a fact noun as Head or not, the fact clause is embedded. Because there is no projecting process involved, to which it could be paratactically or hypotactically related, a fact can appear only in embedded form: either as Qualifier to a ‘fact’ noun, or as a nominalisation on its own (Figure 7-23); for example:
Historically, the fact is [[[that Uncle Tom’s Cabin was the most popular novel of the nineteenth || century and had a huge effect on American history]]].