The Lyman-alpha forest is a series of absorption features in quasar spectra that arise from the distribution of intergalactic gas.  It is a powerful tool for investigating the clustering of matter on scales of 0.5–50 comoving Mpc and has been routinely used to place tight constraints on dark matter models.  In addition, the Lyman-alpha forest is also a calorimeter; the widths of the absorption lines are sensitive to the temperature of intergalactic hydrogen.   I will discuss how this intergalactic calorimeter can be used to test cosmology and the nature of dark matter.  I will focus on how the thermal state of intergalactic gas provides an independent constraint on the cosmic microwave background optical depth [1] that is consistent with the Planck determination, and inconsistent with larger values that have been invoked to evade the recent DESI DR2 tension.  I will also discuss how detailed forward modelling of the Lyman-alpha forest is yielding new constraints on the heating of the intergalactic medium by dark photon dark matter [2, 3].  Intriguingly, dark photons may provide a viable route to resolving the failure of conventional models at reproducing Lyman-alpha absorption line widths arising from low redshift intergalactic gas.