What’s the matter with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi mathvariant="normal">Σ</mml:mi> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>ν</mml:mi> </mml:msub> </mml:math> ?

Because of nonzero neutrino rest masses, we expect the energy density today in nonrelativistic matter, ${\ensuremath{\omega}}_{\mathrm{m}}$, to be greater than the sum of baryon and cold dark matter densities, ${\ensuremath{\omega}}_{\mathrm{cb}}$. We also expect the amplitude of deflections of cosmic microwave background (CMB) photons due to gravitational lensing to be suppressed relative to expectations assuming massless neutrinos. The combination of CMB and baryon acoustic oscillation (BAO) data, however, appears to be defying both of these expectations. Here we review how the neutrino rest mass is determined from cosmological observations, and emphasize the complementary roles played by BAO and lensing data in this process. We then use a phenomenological model to find that the preference from CMB and BAO data for a matter density that is below expectations from the CMB alone is at the $2.3\ensuremath{\sigma}$ level. We also show that, if a fraction of the dark matter decays to dark radiation, the preference for ${\ensuremath{\omega}}_{\mathrm{m}}&gt;{\ensuremath{\omega}}_{\mathrm{cb}}$ can be restored, but with a small increase to the CMB lensing excess.