subsection UNDER RECONSTRUCTION: needs updating

As discussed above in subsection Where is the Baryonic Matter?, most of the baryonic matter in the observable universe is in the intergalactic medium (IGM) (for brevity counting all of the intergalactic medium (IGM), Circumgalactic medium (CGM), and intracluster medium (ICM) as IGM).

Also as discussed above in subsection Where is the Baryonic Matter?, most of the IGM is rather hot.

Why?

The theory is that the gas falls into galaxy superclusters, galaxy clusters from voids and gravitational potential energy gets converted to heat energy. The gas is then the warm-hot intergalactic medium (WHIM) (i.e., ionized H and He gas with temperatures in range 10**5--10**7 K) along with somewhat warmer and colder gas.

Shocks from galaxy collisions and outflows from active galaxies give more heating. Some heat energy may be left from the original phase of galaxy formation. WHIM cools very slowly.

WHIM is almost invisible because it emits low energy X-rays (which are mostly drowned out by Milky Way X-ray emission) and extreme ultraviolet light to which the neutral Milky Way hydrogen is opaque (CO-3).

We have observed WHIM and yours truly thinks it is now established that WHIM is most of the baryonic dark matter.

Eventually, maybe in several Hubble times, some of the WHIM will be cooled enough to collapse into new galaxies (CO-5). This would keep star formation going in the universe for some time. But the continued and accelerated expansion of the universe might prevent all of it from collapsing????.