The standard lambda cold dark matter cosmological model predicts that cosmic filaments are highly clumpy, whereas warm dark matter—invoked to address small-scale challenges in the lambda cold dark matter model—produces filaments that are noticeably smoother and less structured. In this Letter, we investigate the potential of Lyman-alpha emission to trace cosmic filaments at redshifts $z=2.5$ and $z=4$, and assess their potential for constraining the nature of dark matter. Our analysis shows that Lyman-alpha filaments provide a promising observational probe of dark matter: at $z=4$, differences in filament smoothness and surface brightness serve as distinctive signatures between models. Looking ahead, the upcoming generation of 30-m class telescopes will be critical for enabling these measurements, offering a compelling opportunity to distinguish the nature of dark matter by mapping the structure of cosmic filaments.

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• Large scale structure of the Universe
• Particle dark matter
• Astrophysical & cosmological simulations