Newton's cosmology considers the universe (i.e. Milky Way) more-or-less uniformly filled with stars.
In 1755 Immanuel Kant published a book (stimulated by a book by Thomas Wright) in which he described the universe inhabited by a finite Milky Way and surrounded by many similar Milky Ways. No one believed him then. Where is the astronomical evidence?
For centuries astronomers new that in addition to stars, there are so called nebulae on the sky. Some of them were clearly blobs of glowing gas, such as Orion nebula. Most of the astronomers in 18 and 19 centuries believed that all nebulae were indeed glowing blobs of gas.
In 1785 William Herschel (1738-1832) made a first diagram of the Milky Way based on distances to stars that he determined. It was small and amorphous disk of stars.
The main problem was with lack of good ways to measure distances to remote stars and nebulae. Astronomers also did not appreciate that the dust which is present in the space between stars, blocks our view of the central parts of the Milky Way.
In 1912 Vesto Slipher measured Doppler redshifts for some of the spiral nebulae. He found that many of them have velocities much larger than the velocities of stars in the Milky Way.
In 1917 Heber Curtis discovered three faint novae in spiral nebulae. From that he was able to conclude that the spiral nebulae must be many millions light years away. ``Way too far'', said the others.
In 1916 Adriaan van Maanen from the Mount Wilson Observatory (near LA) directly observed the rotational motion of the spiral nebula M100. That was the final blow to the island-universe hypothesis.
At the same time Harlow Shapley from the same Mount Wilson observatory measured the size of the Milky Way using globular clusters. He found that the Milky Way is about 300,000 light years in size. Before him people believed it was only 15,000-20,000 lyrs. The correct number is actually 100,000 lyrs.
If the Milky Way was so enormous as Shapley found, and other spiral nebulae were similar to the Milky Way, then they should be at such large distances, which seem totally implausible. So, the Aristotle prejudice appeared again.
In 1920 the argument culminated in a formal debate between Shapley and Curtis.
Shapley claimed that the Milky Way is very large, and Curtis argued that
the spiral nebulae are indeed external island universes (we call them
now galaxies). Who was right?