Your comment reminds me of a picture I saw a few days ago of a telescope shot, caption "there are no stars in this picture, only galaxies" and there were so. Many. Bright spots.

I don't know where or when it was taken, or what part of the sky that happens in. Maybe it's just a really long lens, so it's seeing "through" the galaxy we normally see "stars" from?

Anyhow, how do you think you could prove this or how someone could prove it? Is it like, two observers on opposite sides of the planet observing the same thing, say during an eclipse or something? Maybe radioastronomy?

You might be thinking of the Hubble Ultra Deep Field image (https://science.nasa.gov/asset/hubble/hubble-ultra-deep-fiel...). I believe it was the result of NASA saying "Let's look at what appears to be a completely dark spot in space, zoom in as far as we can, and see what's there."

We see stars in our galaxy because they are close enough to us that we see them as individual stars. Compare that to the Andromeda Galaxy, which is far enough away that without intense zoom, it looks like a single source of light. There are galaxies even farther away, which we cannot see with the naked eye at all, but zooming in on them like Hubble did means we eventually get enough resolution to see they are individual galaxies, unfathomably far away.

JWST being able to see infrared means we'll see galaxies that are so far away, their light is redshifted so we (and Hubble) cannot even see them at all.

With regards to your question about how to test the bubble hypothesis posted by parent, we would be limited by how variable our point of view can be. We can gather what data we can at one end of Earth's orbit, and then try to see from the opposite end and compare what we see, comparing data sets to see if certain galaxies or stars are in different positions. We already do some of this when dealing with gravitational lensing and I believe it's one of the primary ways we can detect black holes, as they bend light a lot.

I really don't know how you'd prove it, or I'd be operating a cranky social media account demanding the scientific community pay attention to me! My knowledge of astronomy/cosmology tops out around Quanta magazine reader level and studying the subject academically always seemed like a luxury for people who already have money. The 'space is foam' idea just hit me out of the blue one day ~20 years ago when I was staring at bubbles and looking at how tiny ones interact with larger ones. I feel like it has something to do with heliopause and plasma, but every time I read up on it the scientific consensus seems to be 'we still don't know much about it,' so I don't know what to do with the idea.