  Many scenarios of physics beyond the standard model predict new light, weakly
coupled degrees of freedom, populated in the early universe and remaining as
cosmic relics today. Due to their high abundances, these relics can
significantly affect the evolution of the universe. For instance, massless
relics produce a shift $\Delta N_{\rm eff}$ to the cosmic expectation of the
effective number of active neutrinos. Massive relics, on the other hand,
additionally become part of the cosmological dark matter in the later universe,
though their light nature allows them to freely stream out of potential wells.
This produces novel signatures in the large-scale structure (LSS) of the
universe, suppressing matter fluctuations at small scales. We present the first
general search for such light (but massive) relics (LiMRs) with cosmic
microwave background (CMB) and LSS data, scanning the 2D parameter space of
their masses $m_X$ and temperatures $T_X^{(0)}$ today. In the conservative
minimum-temperature ($T_X^{(0)}=0.91$ K) scenario, we rule out Weyl (and
higher-spin) fermions -- such as the gravitino -- with $m_X\geq 2.26$ eV at 95%
C.L., and set analogous limits of $m_X\leq 11.2, 1.06, 1.56$ eV for scalar,
vector, and Dirac-fermion relics. This is the first search for LiMRs with joint
CMB, weak-lensing, and full-shape galaxy data; we demonstrate that weak-lensing
data is critical for breaking parameter degeneracies, while full-shape
information presents a significant boost in constraining power relative to
analyses with only baryon acoustic oscillation parameters. Under the combined
strength of these datasets, our constraints are the tightest and most
comprehensive to date.
