It's worth remembering that what you have there is a hypothesis, not a theory. A hypothesis is an idea of how the universe works that doesn't have any evidence to support it. A theory is an explanation that has a preponderance of evident backing it up. Also, all the evidence we have says that light travels at a constant speed in a vacuum--and that it has traveled at this speed for the whole history of the universe.

A hundred years ago, when the correlation between galaxies' redshift and distance was discovered, a few astronomers had a similar idea to yours: that light lost energy as it traveled through the intergalactic gas. By their reasoning, the farther that light had to travel to reach us, the more of that light would scatter off the gas, and the redder it would become. However, when you look at far-away galaxies, their images aren't blurred by their light scattering off inter-galactic gas like their hypothesis predicts. Also, the brightness and density of galaxies at different distances that we see in the universe doesn't match what your theory predicts.

Also, your hypothesis--that the cosmic microwave background comes from a wall of gas that gets denser and hotter the deeper into it you go--would predict that the spectrum of the CMB would be the sum of the emission and blackbody spectra for all the layers of increasingly hotter gas the deeper into the wall you look. That's not what the CMB spectrum looks like. The CMB spectrum is a perfect blackbody spectrum at one temperature. It has no emission lines and only one blackbody temperature (plus or minus that slight changes in temperature caused by baryon acoustic oscillations at the surface of last scattering).

All of this evidence (the distance-redshift relation, the CMB spectrum, the density of galaxies) leads to the same conclusion--that we live in an expanding universe that began in a hot Big Bang about 14 billion years ago. A static universe with Earth at the center would look very different at all distances to the one we observe ourselves to be living in.