We study the cosmological signatures of invisibly annihilating dark matter (IAnDM), where DM annihilates into dark radiation particles that are decoupled from the Standard Model (SM). In the simple benchmark model we consider here, such invisible annihilation determines the relic abundance of DM via dark thermal freeze-out. We demonstrate that IAnDM may reveal itself through observable, novel signatures that are correlated: scale-dependent $\mathrm{\Delta }{N}_{\mathrm{eff}}$ (number of extra effective neutrinos) in the cosmic microwave background (CMB) spectrum due to DM residual annihilation, while the phase of acoustic peaks shifts towards the opposite direction relative to that due to SM neutrinos, resembling the effect due to scattering (fluidlike) thermal dark radiation; in addition, IAnDM induces modifications to the matter power spectrum that resemble those due to warm dark matter. Current data are sensitive to IAnDM with masses up to $\sim 200\mathrm{keV}$, while future observations will improve this reach, especially if the late-time DM annihilation cross section is enhanced relative to the standard thermal value, which can be realized in a variety of scenarios. This work also demonstrates a new possibility of realizing thermal sub-MeV DM with observable signals.

• Received 15 June 2018

DOI:https://doi.org/10.1103/PhysRevD.100.023004