Keir tells us about constraints on the fraction of dark matter that could be “ultralight” at various masses.
It seems probable now that one single mass of ultralight dark matter (ULDM) can’t be responsible for all the dark matter, but this doesn’t mean it can’t be a sub-component of the dark matter. The structure suppressing properties of ULDM could also have implications for the S8 tension, i.e. maybe a small sub-component of ULDM is what is causing the low S8 in local measurements?
The observational data Keir considers are the Planck CMB data and BOSS clustering data.
Cosmology Talks 