You misunderstand. The universe is very much larger than you imagine.
The cosmic microwave background radiation that we see today has not been
bouncing around in the interstellar gas for the last 13 billion years;
it comes to us directly in a straight line from the edge of the visible
universe.
Many people think Big Bang theory has the initial universe shrunk down to
a very small point. Not so. It was instead a very large volume of matter
with maximum density. For all we know, it might even have been infinitely
large.
If the CMBR came from stars or interstellar dust, it would be stronger
in the galactic plane. It's not.
Rich Daniel rwdaniel@dnaco.net http://www.dnaco.net/~rwdaniel/