  This paper explores the possibility that dark matter particles are objects
with a set of excitable levels, as for a string or drum. A change in excitation
is associated with the absorption or emission of a photon, and the probability
of such a change is dependent on the ambient photon energy density. Particle
mass comes entirely from excitations. Such dark matter with excitable levels,
or DMWEL, particles have a distribution of masses which freezes out to its
present-day form during the early universe. In the particular model considered,
consistency with Planck CMB anisotropy observations implies a cautious lower
bound on the energy of the first excitable level of 2 to 200 eV for a set of
reference parameters. While laboratory detection looks to require a
considerable increase in the power of controlled light-sources, it might be
possible in the near future to detect or constrain DMWEL further through
improved anisotropy observations, closer determination of the CMB's maximum
deviation from a blackbody shape, or from gravitational probes of the dark
matter power spectrum. For certain parameters near to the boundary of the
region allowed by CMB anisotropies, DMWEL dark matter is cold, but with a
cut-off halo mass many orders of magnitude greater than that typical for WIMP
cold dark matter.
