General Relativity was an achievement. But Universe itself is greater, deeper, stranger and different from this achievement. Our best theories … including GR do not correctly ‘predict’ the actual universe. Any such claim is false. Universe also does not laugh on such claims. We ourselves should review our claims whether they are realistic or not.
Before the observation based discovery or confirmation of the existence of galaxies in year 1924, there were three solutions to GR equations available. First by Einstein himself and second by de-Sitter (both: 1917). Both could not reach to the concept of disk shaped island universes. Then in 1922, Friedmann also presented a scheme of various types of universe models. He also failed to reach the concept of disk shaped island universes.
In his 1917 paper, after discussing limitations or problems of Newton’s theory then Einstein proceeds to describe his own theory in section 3. Title of section 3 is following:
3. The Spatially Finite Universe with a Uniform Distribution of Matter
The first paragraph of this section clearly shows that Einstein totally missed the existence of galaxies at large astronomical scales. Following is the first para:
According to the general theory of relativity the metrical character (curvature) of the four-dimensional space-time con- tinuum is defined at every point by the matter at that point and the state of that matter. Therefore, on account of the lack of uniformity in the distribution of matter, the metrical structure of this continuum must necessarily be extremely complicated. But if we are concerned with the structure only on a large scale, we may represent matter to ourselves as being uniformly distributed over enormous spaces, so that its density of distribution is a variable function which varies extremely slowly.
So density varies extremely slowly….
It means that Einstein (or GR) completely missed the concept of island universe having concentrated density and huge voids in-between.
Following is link to English Translation Einstein (1917) paper.
However – Newton did reach to the concept of Galaxies
In a letter to Isaac Newton, David Gregory declared in 1694: “A continual miracle is needed to prevent the Sun and the fixed stars from rushing together through gravity.” Newton pondered the issue over the years starting around 1685 and concluded:
“The fixed stars being… at such vast distances from one another, can neither attract each other perceptibly, nor be attracted by our Sun.” I. Newton, Principia (1728)
Newton reasoned that:
“if the matter of our sun and planets and all the matter in the universe were evenly scattered throughout all the heavens, and every particle had an innate gravity toward all the rest, and the whole space throughout which this matter was scattered was but finite; the matter on the outside of the space would, by its gravity, tend toward all the matter on the inside, and by consequence, fall down into the middle of the whole space and there compose one great spherical mass. But if the matter was evenly disposed throughout an infinite space, it could never convene into one mass; but some of it would convene into one mass and some into another, so as to make an infinite number of great masses, scattered at great distances from one to another throughout all that infinite space. I.Newton, letter to theologian Richard Bentley (1692)
Thus we see that Newton had accurately reached to the idea of the existence of Galaxies and that existence of galaxies actually indicate infinite vastness of space. On the other hand Einstein and his GR had terribly failed in reaching to the concept of galaxies even though observational indications were available at that time and even discussion based on observations regarding existence or non-existence of galaxies was also available.