  There can arise ubiquitous ultra-light scalar fields in the Universe, such as
the pseudo-Goldstone bosons from the spontaneous breaking of an approximate
symmetry, which can make a partial contribution to the dark matter and affect
the large scale structure of the Universe. While the properties of those
ultra-light dark matter are heavily model dependent and can vary in a wide
range, we develop a model-independent analysis to forecast the constraints on
their mass and abundance using futuristic but realistic 21 cm observables as
well as CMB fluctuations, including CMB lensing measurements. Avoiding the
highly nonlinear regime, the 21 cm emission line spectra are most sensitive to
the ultra-light dark matter with mass m ~10^{-26} eV for which the precision
attainable on mass and abundance bounds can be of order of a few percent.
