The distribution of neutral hydrogen (HI) will be mapped over unprecedented volumes of the Universe thanks to the intensity mapping technique with several forthcoming experiments such as the Square Kilometre Array and its South African pathfinder, MeerKAT. Unveiling cosmological information over a wide scale range requires the understanding of the spatial distribution of HI, a biased tracer of dark matter. Most current approaches of the modeling of clustering are constructed to predict a linearly biased power spectrum on large scales. By doing so they ignore the coupling between small and large scale modes during the build-up of large scale structure: density fluctuations in denser environments are enhanced as compared to those in less dense ones. Meanwhile the relation between neutral hydrogen and dark matter is barely known and often assumed to be similar to that of optical galaxies, which might not be the case.
Using a perturbative approach I will show how non-linearities have a significant contribution on linear scales: they modify the expected signature of baryonic acoustic oscillations and these modifications depend on the location of HI in the cosmic web. I will also discuss the observed lack of clustering in the cross-correlation HI x galaxies.