  The Dark Ages, probed by the redshifted 21-cm signal, is the ideal epoch for
a new rigorous test of the standard LCDM cosmological model. Divergences from
that model would indicate new physics, such as dark matter decay (heating) or
baryonic cooling beyond that expected from adiabatic expansion of the Universe.
In the early Universe, most of the baryonic matter was in the form of neutral
hydrogen (HI), detectable via its ground state's spin-flip transition. A
measurement of the redshifted 21-cm spectrum maps the history of the HI gas
through the Dark Ages and Cosmic Dawn and up to the Epoch of Reionization
(EoR). The Experiment to Detect the Global EoR Signature (EDGES) recently
reported an absorption trough at 78 MHz (redshift z of 17), similar in
frequency to expectations for Cosmic Dawn, but about 3 times deeper than was
thought possible from standard cosmology and adiabatic cooling of HI.
Interactions between baryons and slightly-charged dark matter particles with
electron-like mass provide a potential explanation of this difference but other
cooling mechanisms are also being investigated to explain these results. The
Cosmic Dawn trough is affected by cosmology and the complex astrophysical
history of the first luminous objects. Another trough is expected during the
Dark Ages, prior to the formation of the first stars and thus determined
entirely by cosmological phenomena (including dark matter). Observations on or
in orbit above the Moon's farside can investigate this pristine epoch (15-40
MHz; z=100-35), which is inaccessible from Earth. A single cross-dipole antenna
or compact array can measure the amplitude of the 21-cm spectrum to the level
required to distinguish (at >5$\sigma$}) the standard cosmological model from
that of additional cooling derived from current EDGES results. This observation
constitutes a powerful, clean probe of exotic physics in the Dark Ages.
