Question: The authors of model-dependent realism claim that what type of theory may not exist?

Context paragraphs:
Model-centered instruction: Model-centered instruction is a general theory of instructional design developed by Andrew S. Gibbons.  This theory can be used to design individual and group instruction for all kinds of learning in any type of learning environment.  In addition, this theory may be used to design instruction with a wide variety of technologies and many media delivery systems.

Transcendental realism: Initially developed by Roy Bhaskar in his book "A Realist Theory of Science" (1975), transcendental realism is a philosophy of science that was initially developed as an argument against epistemic realism of Positivism and Hermeneutics.  The position is based on Bhaskar's transcendental arguments for certain ontological and epistemological positions based on what reality must be like in order for scientific knowledge to be possible.  While Bhaskar's work is the origin of transcendental realism, it is based upon and further developed by a number of other authors[ref: Collier, Sayer, Lawson, Critical Realism: Essential Readings, etc.  The overview of transcendental realism that follows is largely based on Andrew Sayer's "Realism and Social Science".

N=8 Supergravity: N=8 Supergravity is the most symmetric quantum field theory which involves gravity and a finite number of fields.  It can be found from a dimensional reduction of 11D supergravity by making the size of 7 of the dimensions go to zero.  It has 8 supersymmetries which is the most any gravitational theory can have since there are 8 half-steps between spin 2 and spin -2.  (The spin 2 graviton is the particle with the highest spin in this theory).  More supersymmetries would mean the particles would have superpartners with spins higher than 2.  The only theories with spins higher than 2 which are consistent involve an infinite number of particles (such as String Theory and Higher-Spin Theories).  Stephen Hawking in his Brief History of Time speculated that this theory could be the Theory of Everything.  However, in later years this was abandoned in favour of String Theory.  There has been renewed interest in the 21st century with the possibility that this theory may be finite.

Center versus periphery: Center versus periphery (方言周圏論 , Hōgen-Shūken-Ron , lit.  "Surrounding-zones dialect theory") is a linguistic theory put forward by Japanese folklorist Yanagita Kunio explaining the usage of certain words in a language used in some regions while not in others.  The theory may also explain the existence of other cultural features, or lack thereof, in differing regions; though it was specifically created to address linguistic differences.

Model-dependent realism: Model-dependent realism is a view of scientific inquiry that focuses on the role of scientific models of phenomena.  It claims reality should be interpreted based upon these models, and where several models overlap in describing a particular subject, multiple, equally valid, realities exist.  It claims that it is meaningless to talk about the "true reality" of a model as we can never be "absolutely" certain of anything.  The only meaningful thing is the usefulness of the model.  The term "model-dependent realism" was coined by Stephen Hawking and Leonard Mlodinow in their 2010 book, "The Grand Design".

Banach measure: In mathematics, Banach measure in measure theory may mean a real-valued function on an algebra of all subsets of a set (for example, all subsets of the plane), by means of which a rigid, finitely additive area can be defined for every set, even when a set does not have a true geometric area.  That is, this is a type of generalized measure getting around the phenomenon of non-measurable sets. However, as the Vitali set shows, it cannot be countably additive.

BET theory: Brunauer–Emmett–Teller (BET) theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of materials.  In 1938, Stephen Brunauer, Paul Hugh Emmett, and Edward Teller published the first article about the BET theory in the Journal of the American Chemical Society.  The BET theory applies to systems of multilayer adsorption and usually utilizes probing gases that do not chemically react with material surfaces as adsorbates to quantify specific surface area.  Nitrogen is the most commonly employed gaseous adsorbate used for surface probing by BET methods.  For this reason, standard BET analysis is most often conducted at the boiling temperature of N (77 K).  Further probing adsorbates are also utilized, albeit with lower frequency, allowing the measurement of surface area at different temperatures and measurement scales.  These have included argon, carbon dioxide, and water.  Specific surface area is a scale-dependent property, with no single true value of specific surface area definable, and thus quantities of specific surface area determined through BET theory may depend on the adsorbate molecule utilized and its adsorption cross section.

The Grand Design (book): The Grand Design is a popular-science book written by physicists Stephen Hawking and Leonard Mlodinow and published by Bantam Books in 2010.  The book examines the history of scientific knowledge about the universe and explains 11 dimension M-theory.  The authors of the book point out that a Unified Field Theory (a theory, based on an early model of the universe, proposed by Albert Einstein and other physicists) may not exist.

Defensive realism: Defensive neorealism is a structural theory derived from the school of realism in international relations theory.  It finds its foundation in Kenneth Waltz's "Theory of International Politics", in which Waltz argues that the anarchical structure of the international system encourages states to maintain moderate and reserved policies to attain security.  In contrast, offensive realism assumes that states seek to maximize their power and influence to achieve security through domination and hegemony.  Defensive neorealism asserts that aggressive expansion as promoted by offensive neorealists upsets the tendency of states to conform to the balance of power theory, thereby decreasing the primary objective of the state, which they argue is ensuring its security.  While defensive realism does not deny the reality of interstate conflict, nor that incentives for state expansion do exist, it contends that these incentives are sporadic rather than endemic.  Defensive neorealism points towards "structural modifiers" such as the security dilemma and geography, and elite beliefs and perceptions to explain the outbreak of conflict.

Economic Justice: Justice in economics is a subcategory of welfare economics with models frequently representing the ethical-social requirements of a given theory, whether "in the large", as of a just social order, or "in the small", as in the equity of "how institutions distribute specific benefits and burdens".  That theory may or may not elicit acceptance.  In the Journal of Economic Literature classification codes 'justice' is scrolled to at , wedged on the same line between 'Equity' and 'Inequality' along with 'Other Normative Criteria and Measurement'.  Categories above and below the line are Externalities and Altruism.

Supporting paragraphs: Model-dependent realism The Grand Design (book)
Answer: Unified Field Theory