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Ubiquitous ultra-light scalar fields may make a partial contribution to the dark matter and affect the large scale structure of the Universe. While their properties are heavily model dependent, we develop a model-independent analysis to forecast the constraints on their mass and abundance using futuristic 21~cm observation as well as CMB lensing measurements. We demonstrate that the 21~cm power spectrum 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 the order of a few percent.
Kadota et al. (Mon,) studied this question.