We present a comprehensive analysis of the sensitivity of atomic transitions to light dark matter pair absorption. Unlike scattering, where only a fraction of the dark matter energy is deposited, pair absorption processes absorb the full mass and are therefore capable of constraining far lighter dark matter. Spin-flip and fine-structure transitions are able to constrain electroweak scale axial-vector couplings for bosonic dark matter with $\mathrm{\mu }\mathrm{eV}$ to eV masses, while principal quantum number transitions are able to set similar constraints on bosonic dark matter with scalar couplings. Unfortunately, pair absorption is largely insensitive to light fermionic dark matter due to its necessarily tiny density at low masses. We also demonstrate the sensitivity of pair absorption to the cosmic neutrino background and find that spin-flip transitions can set constraints on the overdensity parameter of ${\eta }_{\nu }\lesssim {10}^9$ for neutrino masses $m_{\nu }\lesssim 1\mathrm{meV}$, around 100 times stronger than existing constraints.

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• Cosmology
• Dark matter direct detection
• Neutrino detection