 When we look out into the universe far enough, we see, on average, the same distribution of galaxies in all directions. In other words, there is no preferred direction in space, it's isotropic. But being isotropic for us doesn't necessarily mean it is isotropic for viewers in far-off locations. It is possible to imagine galaxy configurations, where things look the same in all directions from one point of view and not from another. But this is unlikely. Our assumption is that all observers will see the same isotropic distribution of galaxies. In other words, there is no preferred location in the universe, it's homogeneous. Galaxy densities are the same everywhere as long as we are viewing large enough distances and volumes. This is the cosmological principle. It simply states that the universe is both isotropic with no preferred direction and homogeneous with no preferred place. All directions are equal, and all places are equal as far as the laws of physics are concerned. But we know that galaxies clump up into galaxy clusters, and that these galaxy clusters clump up into superclusters. So to get to this cosmological principle, we need to be talking about distances of many billions of light years. For comparison, it's like saying the surface of the earth is a sphere, but up close we see mountains and valleys galore. It isn't clear that we have a sphere until we get far enough away for the nearby structures to average out. We have to go out 45,000 kilometers to actually see the sphere.