Understanding the nature of dark matter (DM) particles remains a pivotal challenge in modern cosmology. Current cosmological research on these phenomena primarily utilizes early-universe cosmic microwave background (CMB) observations and other late-time probes, which predominantly focus on large scales. We introduce a novel probe, the 21 cm forest signal, which can be used to investigate DM properties on small scales during the epoch of reionization, thereby addressing the gap left by other cosmological probes. Annihilation and decay of DM particles, as well as Hawking radiation from primordial black holes (PBHs), can heat the intergalactic medium (IGM). This heating suppresses the amplitude of the 21 cm forest 1D power spectrum. Therefore, the 1D power spectrum provides an effective method for constraining DM properties. However, astrophysical heating processes in the early Universe can also affect the 21 cm forest 1D power spectrum. In this work, we assess the potential of using the Square Kilometre Array (SKA) to observe the 21 cm forest 1D power spectrum for constraining DM properties, under the assumption that astrophysical heating can be constrained reliably by other independent probes. Under low astrophysical heating conditions, the 1D power spectrum could constrain the DM annihilation cross section and decay lifetime to $⟨\sigma v⟩\sim {10}^{-31}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ and $\tau \sim {10}^{30}\mathrm{s}$ for 10 GeV DM particles, and probe PBHs with masses $\sim {10}^{15}\mathrm{g}$ at abundances $f_{\mathrm{PBH}}\simeq {10}^{-13}$. These constraints represent improvements of 5-6 orders of magnitude over current limits. Furthermore, the 21 cm forest 1D power spectrum has the potential to exceed existing bounds on sub-GeV DM and to probe PBHs with masses above ${10}^{18}\mathrm{g}$, which are otherwise inaccessible by conventional cosmological probes. With accumulating observational data and technological advancements, the 21 cm forest emerges as a highly promising tool for probing DM properties.

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• Dark matter
• Large scale structure of the Universe