Question: It was believed that the celestial region was made of a fundamentally different kind of matter, who thought it was made of Aether?
About the article "Astrophysics", section "History".

Candidate passages:
In Plato's "Timaeus" (55d) speaking about air, Plato mentions that "there is the most translucent kind which is called by the name of aether (αίθηρ)".
Aristotle, who had been Plato's student at the Akademia, disagreed with his former mentor and added "aether" to the system of the classical elements of Ionian philosophy as the "fifth element", He noted that the four terrestrial classical elements were subject to change and naturally moved linearly.
Aether however, located in the celestial regions and heavenly bodies, moved circularly.
In Aristotle's system of classical elements, aether had none of the qualities the terrestrial classical elements had.
Aether was neither hot nor cold, neither wet nor dry.
Aether did not follow Aristotelian physics either.
Aether was also incapable of motion of quality or motion of quantity.
Aether was only capable of local motion.
Aether naturally moved in circles, and had no contrary, or unnatural, motion.
Aristotle also noted that crystalline spheres made of aether held the celestial bodies.
The idea of crystalline spheres and natural circular motion of aether led to Aristotle's explanation of the observed orbits of stars and planets in perfectly circular motion in crystalline aether.
Medieval scholastic philosophers granted "aether" changes of density, in which the bodies of the planets were considered to be more dense than the medium which filled the rest of the universe.
Robert Fludd stated that the aether was of the character that it was "subtler than light".
Fludd cites the 3rd-century view of Plotinus, concerning the aether as penetrative and non-material.
See also Arche. 
Although astronomy is as ancient as recorded history itself, it was long separated from the study of terrestrial physics.
In the Aristotelian worldview, bodies in the sky appeared to be unchanging spheres whose only motion was uniform motion in a circle, while the earthly world was the realm which underwent growth and decay and in which natural motion was in a straight line and ended when the moving object reached its goal.
Consequently, it was held that the celestial region was made of a fundamentally different kind of matter from that found in the terrestrial sphere; either Fire as maintained by Plato, or Aether as maintained by Aristotle.
In the 17th century, natural philosophers such as Galileo, Descartes, and Newton began to maintain that the celestial and terrestrial regions were made of similar kinds of material and were subject to the same natural laws.
At the end of the 19th century, it was discovered that, when decomposing the light from the Sun, a multitude of spectral lines were observed (regions where there was less or no light).
Laboratory experiments with hot gases showed that the same lines could be observed in the spectra of known gases, specific lines corresponding to unique chemical elements.
In this way it was proved that the chemical elements found in the Sun and stars (chiefly hydrogen) were also found on Earth.
Indeed, the element helium was first discovered in the spectrum of the Sun and only later found on Earth, hence its name.
During the 20th century, spectroscopy (the study of these spectral lines) advanced, particularly as a result of the advent of quantum physics that was necessary to understand the astronomical and experimental observations.
See also:
 Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure
 Timeline of white dwarfs, neutron stars, and supernovae
 Timeline of black hole physics
 Timeline of gravitational physics and relativity
Before 1887 it was generally believed that light travelled at a constant speed relative to the hypothesised medium of the aether.
For an observer in motion with respect to the aether, this would result in slightly different two-way speeds of light in different directions.
In 1887, the Michelson–Morley experiment showed that the two-way speed of light was constant regardless of direction or motion through the aether. 
In 1916, after Einstein completed his foundational work on general relativity, Lorentz wrote a letter to him in which he speculated that within general relativity the aether was re-introduced.
In his response Einstein wrote that one can actually speak about a "new aether", but one may not speak of motion in relation to that aether.
This was further elaborated by Einstein in some semi-popular articles (1918, 1920, 1924, 1930).
In 1918 Einstein publicly alluded to that new definition for the first time.
Then, in the early 1920s, in a lecture which he was invited to give at Lorentz's university in Leiden, Einstein sought to reconcile the theory of relativity with his mentor's cherished concept of the aether.
In this lecture Einstein stressed that special relativity took away the last mechanical property of Lorentz's aether: immobility.
However, he continued that special relativity does not necessarily rule out the aether, because the latter can be used to give physical reality to acceleration and rotation.
This concept was fully elaborated within general relativity, in which physical properties (which are partially determined by matter) are attributed to space, but no substance or state of motion can be attributed to that "aether" (aether = curved space-time).
In another paper of 1924, named "Concerning the Aether", Einstein argued that Newton's absolute space, in which acceleration is absolute, is the "Aether of Mechanics".
And within the electromagnetic theory of Maxwell and Lorentz one can speak of the "Aether of Electrodynamics", in which the aether possesses an absolute state of motion.
As regards special relativity, also in this theory acceleration is absolute as in Newton's mechanics.
However, the difference from the electromagnetic aether of Maxwell and Lorentz lies in the fact, that ""because it was no longer possible to speak, in any absolute sense, of simultaneous states at different locations in the aether, the aether became, as it were, four dimensional, since there was no objective way of ordering its states by time alone."".
Now the "aether of special relativity" is still "absolute", because matter is affected by the properties of the aether, but the aether is not affected by the presence of matter.
This asymmetry was solved within general relativity.
Einstein explained that the "aether of general relativity" is not absolute, because matter is influenced by the aether, just as matter influences the structure of the aether.
So the only similarity of this relativistic aether concept with the classical aether models lies in the presence of physical properties in space.
Therefore, as historians such as John Stachel argue, Einstein's views on the "new aether" are not in conflict with his abandonment of the aether in 1905.
For, as Einstein himself pointed out, no "substance" and no state of motion can be attributed to that new aether.
In addition, Einstein's use of the word "aether" found little support in the scientific community, and played no role in the continuing development of modern physics. 
In considering the physics of the celestial spheres, scholars followed two different views about the material composition of the celestial spheres.
For Plato, the celestial regions were made "mostly out of fire" on account of fire's mobility.
Later Platonists, such as Plotinus, maintained that although fire moves naturally upward in a straight line toward its natural place at the periphery of the universe, when it arrived there, it would either rest or move naturally in a circle.
This account was compatible with Aristotle's meteorology of a fiery region in the upper air, dragged along underneath the circular motion of the lunar sphere.
For Aristotle, however, the spheres themselves were made entirely of a special fifth element, "Aether" (Αἰθήρ), the bright, untainted upper atmosphere in which the gods dwell, as distinct from the dense lower atmosphere, "Aer" (Ἀήρ).
While the four terrestrial elements (earth, water, air and fire) gave rise to the generation and corruption of natural substances by their mutual transformations, aether was unchanging, moving always with a uniform circular motion that was uniquely suited to the celestial spheres, which were eternal.
Earth and water had a natural heaviness ("gravitas"), which they expressed by moving downward toward the center of the universe.
Fire and air had a natural lightness ("levitas"), such that they moved upward, away from the center.
Aether, being neither heavy nor light, moved naturally around the center.