Dear Steve Willner:

On Wednesday, October 19, 2016 at 12:15:35 PM UTC-7, Steve Willner wrote:
In article ,
dlzc writes:
....
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.

Individually, no. But in groups, with a center of momentum frame, they do have rest mass. And hence gravitational 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.

As I said, I did not expect it to be even 1%, much less what is required to be Dark Matter.

Thank you.

David A. Smith