The evidence that Dark Matter is made of hydrogen at its ground state has increased these last 2 years from so-called 21-centimeter cosmology, see here for example:
Interestingly, UDH as main component of DM does still better fulfill current observational constraints on DM, especially the one on cosmic dawn cosmic microwave background (CMB) decoupling, see e.g. this excellent study
These guys (https://arxiv.org/abs/1803.06698v1) use fringe approximations to calculate the 21.1cm line. In the first reference even once the wrong wave number is given. Please look it up at Mills chpt: 2.227!