  We propose a matter effect for the gravitational ultraviolet/infrared (UV/IR)
mixing solution to the cosmological constant problem. Previously, the
gravitational UV/IR mixing model implied a non-standard equation of state for
dark energy, contradicting observation. In contrast, matter effect
gravitational UV/IR mixing accommodates a standard $\Lambda$CDM cosmology with
constant dark energy. Notably, there are new density-dependent predictions for
futuristically precise measurements of fundamental parameters, like the
magnetic moments of the muon and electron.
