But regarding the letter of the professor who says:
What matter could remain matter (i.e. atomic bonds would remain intact!) at that heat and force and not evaoprate and be turn into pure energey?
These are all conjectures. Just as it is a conjecture that there is dark matter (but it makes the formula work just right). If the space has background tamperature, and is not absolute zero, it means it has matter in it. If so, the bodies flying through the space at high velocities would collide with that matter all the time, resulting in light or some other readiomagnetic radiation being emitted (and detected).
Areas where there is matter, we have evidence of light, such as various hydrogen clouds collapsing towards a center of gravity as they form stars. The glow resulting from collinsions of atoms at high speed appears as light (greenish to the eye, but mostly in the invisible red spectrum captured on film).
Post 12,897 was attempting to explain the graph a bit further.