  Dark matter may be hidden, with no standard model gauge interactions. At the
same time, in WIMPless models with hidden matter masses proportional to hidden
gauge couplings squared, the hidden dark matter's thermal relic density may
naturally be in the right range, preserving the key quantitative virtue of
WIMPs. We consider this possibility in detail. We first determine
model-independent constraints on hidden sectors from Big Bang nucleosynthesis
and the cosmic microwave background. Contrary to conventional wisdom, large
hidden sectors are easily accommodated. A flavour-free version of the standard
model is allowed if the hidden sector is just 30% colder than the observable
sector after reheating. Alternatively, if the hidden sector contains a
1-generation version of the standard model with characteristic mass scale below
1 MeV, even identical reheating temperatures are allowed. We then analyze
hidden sector freezeout in detail for a concrete model, solving the Boltzmann
equation numerically and understanding the results from both observable and
hidden sector points of view. We find that WIMPless dark matter indeed obtains
the correct relic density for masses in the range keV < m_X < TeV. The upper
bound results from the requirement of perturbativity, and the lower bound
assumes that the observable and hidden sectors reheat to the same temperature
and is raised to the MeV scale if the hidden sector is 10 times colder.
WIMPless dark matter therefore generalizes the WIMP paradigm to the largest
mass range possible for viable thermal relics and provides a unified framework
for exploring dark matter signals across nine orders of magnitude in dark
matter mass.
