Structures in the Universe grew through gravitational instability from very smooth initial conditions. Energy conservation requires that the growing negative potential energy of these structures be balanced by an increase in kinetic energy. A fraction of this is converted into heat in the collisional gas of the intergalactic medium. Using a toy model of gravitational heating, we attempt to link the growth of structure in the Universe with the average temperature of this gas.We find that the gas is rapidly heated from collapsing structures at around z 10, reaching a temperature >106 K today, depending on some assumptions of our simplified model. Before that there was a cold era from z 100 to 10 in which the matter temperature was below that of the cosmic microwave background.
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