The distribution of hydrogen gas was inferred using Lyman-alpha forest observations from Hubble Space Telescope's Cosmic Origins Spectrograph.
A team led by Juna Kollmeier reported an unexpected deficit of roughly 400% between ionizing light from known sources and the actual observations of intergalactic hydrogen.
It's possible the simulations do not reflect reality, which by itself would be a surprise, because intergalactic hydrogen is the component of the Universe that we think we understand the best.”[1] Kollmeier and her team state that "... either conventional sources of ionizing photons (galaxies and quasars) must contribute considerably more than current observational estimates or our theoretical understanding of the low-redshift universe is in need of substantial revision.”[4] A similar study, led by Michael Shull, found that the deficit is only twice as large and not five times larger, as previously claimed.
Khaire & Srianand [6] showed that a metagalactic photoionization rate that is two to five times larger can be easily obtained using updated quasar and galaxy observations.
Including AGN feedback was shown to be an important element for heating in the low redshift intergalactic medium (IGM) (Gurvich, Burkhart, & Bird 2016.[7]).