  Precise measurements of the temperature and polarization anisotropies of the
cosmic microwave background can be used to constrain the annihilation and decay
of dark matter. In this work, we demonstrate via principal component analysis
that the imprint of dark matter decay on the cosmic microwave background can be
approximately parameterized by a single number for any given dark matter model.
We develop a simple prescription for computing this model-dependent
detectability factor, and demonstrate how this approach can be used to set
model-independent bounds on a large class of decaying dark matter scenarios. We
repeat our analysis for decay lifetimes shorter than the age of the universe,
allowing us to set constraints on metastable species other than the dark matter
decaying at early times, and decays that only liberate a tiny fraction of the
dark matter mass energy. We set precise bounds and validate our principal
component analysis using a Markov Chain Monte Carlo approach and Planck 2015
data.
