The Bright Future of the Dark and Dim Universe

Abstract

This chapter investigates the low-mass frontier of galaxy formation through two complementary populations: the starless Reionization-Limited HI Clouds (RELHICs) that trace the ``dark'' Universe, and the faint, gas-rich galaxies that define the ``dim'' Universe. RELHICs offer pristine laboratories for probing the distribution of DM on sub-galactic scales, providing a direct test of the Lambda Cold Dark Matter ($Λ$CDM) model predictions. The dim Universe provides statistical constraints on cosmology, galaxy formation and evolution, as well as baryoninc physics through key observables including the low-mass end of the neutral-hydrogen mass function (HIMF), the neutral-hydrogen velocity function (HIVF), and the low-mass end of the baryonic Tully-Fisher relation (bTFR). This chapter outlines core science questions that can be tackled leveraging radio observations of both the dark and dim Universe. Additionally, it outlines strategies to identify RELHICs amid tidal or pressure-confined contaminants, while providing observational predictions for the dim Universe. The Square Kilometre Array (SKA) in its mid-frequency Array Assembly 4 (AA4) configuration will, for the first time, resolve the internal gas structure of nearby RELHICs and build deep, wide-area datasets that definitively constrain the HIMF, HIVF, and bTFR down to masses of $10^{6}~\msol$ -- offering a complete observational framework to test the $Λ$CDM paradigm and the baryonic processes that shape the faint end of galaxy formation.