[Pontiac]

The missing part of this matter was thought to be a hot, ultra-thin gas haze of very low density between larger structures. Its hellacious temperature means that it only emits far-ultraviolet and X-ray radiation.

A similar baryonic haze, 150 times hotter than the sun's surface, was indirectly detected surrounding the Milky Way

Puzzling

What keeps this ultra-thin gas haze of very low density so hot for 13 billion years?

One would think that only fusion could do this. But Physics as I understand it would require enormous pressures to sustain fusion reaction. Such pressures do not exist in the vacuum of space.

There must be physics at work here that we do not understand. Once we do I think our energy crisis is over.

[Telepathic Intruder]

Apparently it is absorbing x-rays and re-emitting them as black body radiation. The temperature is based on the wavelenth.
I would think that only the missing matter in galactic halos are hot, and the "fillaments" between galaxies cooler. According to the simulations its like a big cobweb, with galaxies forming at the intersections.

[Hoosier-Daddy]

Vacuum. No conduction of heat. Think REALLY LARGE Nissan Stainless.