  We study dark matter-helium scattering in the early Universe and its impact
on constraints from cosmic microwave background (CMB) anisotropy measurements.
We describe possible theoretical frameworks for dark matter-nucleon
interactions via a scalar, pseudoscalar, or vector mediator; such interactions
give rise to hydrogen and helium scattering, with cross sections that have a
power-law dependence on relative velocity. Within these frameworks, we consider
three scenarios: dark matter coupling to only neutrons, to only protons, and to
neutrons and protons with equal strength. For these various cases, we use
\textit{Planck} 2018 temperature, polarization, and lensing anisotropy data to
place constraints on dark matter scattering with hydrogen and/or helium for
dark matter masses between 10 keV and 1 TeV. For any model that permits both
helium and hydrogen scattering with a non-negative power-law velocity
dependence, we find that helium scattering dominates the constraint for dark
matter masses well above the proton mass. Furthermore, we place the first CMB
constraints on dark matter that scatters dominantly/exclusively with helium in
the early Universe.
