In article ,
dlzc writes:
I find it more likely that a nearly 100 year old assumption that
luminosity= is directly proportional to the amount of mass
present, when it has long b= een known that luminosity drops off
rapidly with surface temperature.

As you say, the dependence of luminosity on temperature -- more
generally on stellar population -- is well known. It is taken into
account as well as possible given the data available, and the
resulting uncertainties are understood.

If y= ou have cooler objects, they simply don't put out as much
light... especially in the visible light bands.

As you indicate, working in the infrared helps quite a bit. It
doesn't eliminate the uncertainties altogether, though.

Now what I wonder is, if the "perfectly mirrored, massless box,
containing = photons", which has rest mass, exists between a star

Photons have energy, which contributes to gravitation, but they don't
have rest mass.

and the gases/dust/pla= nets that give that star a background
temperature higher than the CMBR. So= some Dark Matter (probably
less than 1%) might still be photons in transit= between
intersystem objects...?

Light has to be considered separately from matter in the cosmological
equations because its energy density decreases as the fourth power of
scale factor. The energy density of light has been less than that of
matter since the first several minutes of cosmic time, and its energy
density is negligible at later epochs.

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