We study a dark matter scenario with QCD-like confinement (QCD-DM), in which a confining non-Abelian gauge sector produces composite dark particles that can decay, injecting energy into the primordial plasma. We analyze the resulting cosmic microwave background (CMB) spectral distortions (SDs) generated by this energy release. We adopt a unified framework capable of describing both relativistic and nonrelativistic particles, as well as slow and fast decay regimes. Within this approach, we study exponential, power-law, oscillatory, and two-step decay processes, computing the resulting $\mu$- and $y$-type distortions across the parameter space spanned by the confinement scale $a_c$, decay rate ${\mathrm{\Gamma }}_{\chi }$, energy-transfer efficiency ${\mathrm{\Sigma }}_{\chi }$, and velocity dispersion $v_{\chi c}$. We find that power-law, oscillatory, and cascade decays can be effectively mapped onto exponential models after appropriate rescaling. The dominant factors controlling SDs are the decay epoch and lifetime, with $v_{\chi c}$ only becoming relevant in the ultrarelativistic limit. FIRAS observations impose constraints on early energy injection, with $\mu$-type distortions providing the tightest bounds on ${\mathrm{\Sigma }}_{\chi }$. Simultaneous matching of both ${\mu }_{\text{firas}}$ and $y_{\text{firas}}$ breaks the degeneracy between ${\mathrm{\Gamma }}_{\chi }$ and ${\mathrm{\Sigma }}_{\chi }$, localizing preferred decay rates around ${\mathrm{\Gamma }}_{\chi }\lesssim (3.3–4.4)\times {10}^{-3}{\mathrm{yr}}^{-1}$ and ${\mathrm{\Sigma }}_{\chi }\lesssim 8.5\times {10}^{-4}$ for relativistic particles, and revealing that fast decays with ${\mathrm{\Gamma }}_{\chi }\gtrsim 6.5{\mathrm{yr}}^{-1}$ or large ${\mathrm{\Sigma }}_{\chi }$ become observationally negligible. Our extended analysis confirms that steep power-law decays shift distortion epochs earlier and strengthen upper limits on ${\mathrm{\Sigma }}_{\chi }$. Overall, our results show that CMB spectral distortions are a powerful probe of dark-sector confinement and decay. Future missions, such as PIXIE and PRISM, could extend current sensitivity by several orders of magnitude and test regions of parameter space that are currently unconstrained, offering a direct observational window into nonstandard early-Universe dynamics.

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• Cosmic microwave background
• Dark matter
• Particle dark matter