By the middle of the 20th century, telescopic observations had revealed the Universe was filled with galaxies. Before Edwin Hubble’s ground-breaking discoveries, it was thought that our own Milky Way Galaxy was the “universe” and those faint smudges (revealed by Hubble to be other galaxies) were clouds of gas in the Milky Way.
Since this revolution in our perception of the cosmos, it has been assumed the Universe is:
isotropic: looks the same in all directions
homogeneous: is the same at all locations (in terms of matter distribution)
It’s important to note that these concepts only apply at a sufficiently large scale. Obviously, if you look in the direction of the Moon, and then look in the opposite direction, things will appear very different indeed. But if you were floating in space between clusters of galaxies, the view would be far more isotropic.
Likewise, the Universe would not appear homogeneous at the scale of the Solar System or Milky Way. But at larger scales, things are “mixed” together fairly well. At least that’s what we thought.
In the last few decades new imaging technology has changed that paradigm. Isotropy and homogeneity are still valid at the largest scales, but ...