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

Dark Cosmology: Investigating Dark Matter & Exotic Physics in the Dark Ages using the Redshifted 21-cm Global Spectrum