The assumption that photons are massless is a foundational postulate of modern physics, yet it remains subject to experimental verification. Fast radio bursts (FRBs), with their cosmological distances and precisely measured dispersion, offer an excellent laboratory for testing this hypothesis. In this work, we propose an improved distribution function for the dispersion measure arising from extragalactic gas and demonstrate that it provides an excellent fit to mock data. We then apply this distribution to constrain the photon rest mass under the $\mathrm{\Lambda }$ cold dark matter (CDM), $w\mathrm{CDM}$, and $w_0{w}_a\mathrm{CDM}$ cosmological models, the last of which is favored by recent Dark Energy Spectroscopic Instrument baryon acoustic oscillation observations. The corresponding $1\sigma$ upper limits on the photon mass are found to be $4.83\times {10}^{-51}\mathrm{kg}$, $4.71\times {10}^{-51}\mathrm{kg}$, and $4.86\times {10}^{-51}\mathrm{kg}$, respectively, which are the most stringent constraints derived from FRBs to date. These results demonstrate that FRBs provide robust and reliable constraints, and offer strong empirical support for the massless nature of the photon.

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• Cosmology
• Transient & explosive astronomical phenomena