I will review the use of neutral hydrogen (HI) in the
post reionization era as a tracer of the structure formation
process. I will focus on atomic hydrogen both in absorption (IGM) and
in emission (21cm intensity mapping). IGM is a probe of HI in volume,
while intensity mapping is sensitive to the HI mass distribution.
This combined information is thus highly complementary.
The IGM, diffuse matter between galaxies (the so-called cosmic web)
allows to measure the matter perturbations down to small scales and at
high redshift. In recent years, baryonic acoustic oscillations have
been detected at high redshift and unprecedented tight constraints on
dark matter nature and neutrino mass have also been obtained, using
IGM data (mainly from low resolution BOSS and high resolution quasar
spectra from HIRES/Keck). On the other hand, emission at 21cm will
probe is going to be an important probe that will also probe structure
formation at high redshift. The IGM and intensity mapping can be used to address fundamental
physical questions like the nature of dark matter, neutrino masses,
possible extension of the standard cosmological model, and offer
important clues on the cosmic cycle of baryons and the galaxy/IGM
interplay.