In popular science (SciPop) the reason why we can detect a distant brightness is because opposing forces, the solar wind pushing out and interstellar forces pushing in, cause glowing Hydrogen to accumulate, like foam on a beach.
The Heliopause
Empirical Evidence of the Firmament
Let's look at empirical evidence of the firmament. That means direct observation. The most recent confirmation that we have of the presence of the firmament is from the New Horizons space mission.
Hypothesis 3
The image here was taken by Voyager 1 by aiming its cameras back towards the Earth and taking a series of pictures. It is called the Solar System Family Portrait.
Voyager Found the Firmament
The Voyager spacecraft have been on their missions since 1977 and have flown far past the orbits of the planets in our solar system. They took a solar system family portrait and found the firmament.