Abstract: The standard paradigm of collisionless cold dark matter is
in tension with measurements on large scales. In particular, the best
fit values of the Hubble rate H_0 and the matter density perturbation
sigma_8 inferred from the cosmic microwave background seem
inconsistent with the results from direct measurements. We show that
both problems can be solved in a framework in which dark matter
consists of two distinct components, a dominant component and a
subdominant component. The primary component is cold and
collisionless. The secondary component is also cold, but interacts
strongly with dark radiation, which itself forms a tightly coupled
fluid. The growth of density perturbations in the subdominant
component is inhibited by dark acoustic oscillations due to its
coupling to the dark radiation, solving the sigma_8 problem, while the
presence of tightly coupled dark radiation ameliorates the H_0
problem. The subdominant component of dark matter and dark radiation
continue to remain in thermal equilibrium until late times, inhibiting
the formation of a dark disk. We present an example of a simple model
that naturally realizes this scenario in which both constituents of
dark matter are thermal WIMPs. Our scenario can be tested by future
stage-IV experiments designed to probe the CMB and large scale
structure.