  The present-day Universe is seemingly dominated by dark energy and dark
matter, but mapping the normal (baryonic) content remains vital for both
astrophysics - understanding how galaxies form - and astro-particle physics -
inferring properties of the dark components. The Square Kilometre Array (SKA)
will provide the only means of studying the cosmic evolution of neutral
Hydrogen (HI) which, alongside information on star formation from the radio
continuum, is needed to understand how stars formed from gas within dark-matter
over-densities and the roles of gas accretion and galaxy merging. `All
hemisphere' HI redshift surveys to redshift 1.5 are feasible with
wide-field-of-view realizations of the SKA and, by measuring the galaxy power
spectrum in exquisite detail, will allow the first precise studies of the
equation-of-state of dark energy. The SKA will be capable of other uniquely
powerful cosmological studies including the measurement of the dark-matter
power spectrum using weak gravitational lensing, and the precise measurement of
H0 using extragalactic water masers. The SKA is likely to become the premier
dark-energy-measuring machine, bringing breakthroughs in cosmology beyond those
likely to be made possible by combining CMB (e.g. Planck), optical (e.g. LSST,
SNAP) and other early-21st-century datasets.
