The theoretical framework of Kaluza-Klein (KK) theory, which posits the existence of extra spatial dimensions, is finding new relevance in addressing the persistent challenge of dark matter. The concept originated with Theodor Kaluza's 1919 hypothesis, which sought to unify gravity and electromagnetism by incorporating a compact fifth dimension. This foundational work has evolved significantly, particularly through the introduction of Randall-Sundrum (RS) models in 1999 by Lisa Randall and Raman Sundrum.
The RS models utilize a five-dimensional Anti-de Sitter (AdS) spacetime characterized by warped geometry. This warping mechanism offers a theoretical resolution to the hierarchy problem—the vast disparity, approximately 10^24, between the strength of gravity and the other fundamental forces, as noted in recent research by scientists from Spain and Germany. The warping allows the extra dimensions to influence gravity's perceived weakness while suppressing observable effects on the four-dimensional 'brane' where Standard Model particles reside.
While early concepts suggested extra dimensions might be as large as one-tenth of a millimeter, experimental searches at facilities like the Large Hadron Collider (LHC) as of late 2025 have not confirmed the existence of the massive, graviton-like particles predicted by these theories, thereby constraining hypothetical mass ranges. Despite this, a theoretical development published in The European Physical Journal C in November 2025 proposes a compelling new path connecting this framework to cosmology.
The November 2025 study suggests that fermions propagating within a warped fifth dimension could manifest as the elusive dark matter, which accounts for approximately 75 percent of the universe's total matter content. This research specifically examines how fermion masses could be communicated into the fifth dimension via theoretical 'portals,' generating 'fermionic dark matter' relics. Furthermore, related theoretical work explores the possibility that massive gravitational states, or KK modes, arising from Kaluza-Klein gravity itself, could serve as dark matter candidates.
The viability of these scenarios is currently under intense scrutiny. While some models, such as those involving spin-2 KK portal dark matter, show that scalar and fermion dark matter models are highly constrained under standard thermal freeze-out conditions, vector dark matter models remain viable within specific parameter spaces. The potential detection of this hypothesized fermionic dark matter, according to the November 2025 findings, may now depend on advancements in gravitational wave detection technology, marking a strategic shift from collider searches to gravitational astronomy for testing physics beyond the Standard Model.
