What is in space?
There is no justification to define limits on the universe so it must treated as an infinite volume of space.
This infinite volume was not created. This infinite space is not a thing. The creation of something requires it to be separate from other things which were also created. One infinite thing covers everything.
Space contains everything we can see and measure. Space is not a physical thing having a size.
If someone claims the universe MUST be a distinct infinite thing, then it is illogical and meaningless to claim there is an infinite thing separate from infinite space.
This infinite space is definitely not empty. There are many galaxies and quasars at varying distances, beyond the millions of stars and nebulae in our Milky Way.
The space between those visible objects is also not empty.
This was known by the 1930's but not explicitly described that way.
The infinite space has stuff in it and this space has apparent partitions of matter and plasma.
In 1936, Edwin Hubble concluded our Local Group is separate from the Hubble Flow; essentially our Local Group is in a partition (about 8Mly in diameter) separate from the assumed expanding universe.
Though spectra from that time are not available, this is what I expect he had to reach his conclusion:
NASA has a page with the M31 absorption lines for an exercise for a student so one can assume Hubble saw this spectrum also.
a) M31 and M33 had blue shifted calcium absorption lines,
b) both Magellanic Clouds had red shifted calcium absorption lines,
c) most of the galaxies beyond our Local Group had red shifted hydrogen absorption lines.
d) Cepheid variables in the Magellanic Clouds were used by Henrietta Swan Leavitt to define their behavior, in the 1920's.
e) in 1922, Hubble found a Cepheid in M31 to allow a distance calculation, placing it more than 2 million light years away.
f) This was the first evidence for galaxies beyond our Milky Way.
g) with more of the distant galaxies being measured, Hubble discovered a correlation.
h) Red shifts in the hydrogen absorption line increased roughly proportional to its Cepheid distance.
i) M31, M33, and the Magellanic Clouds did not follow this correlation.
j) a negative velocity results in a negative distance which is impossible,
k) the Magellanic Cloud red shifts using the hydrogen red shift proportion to distance calculation placed them near M31 which is impossible when they are visually so close (and a closer distance was found by Cepheids).
Therefore Hubble put our Local Group in a partition separate from the Hubble Flow.
The better solution to the sequence of (a) to (k) is explain the discrepancy rather than ignore it.
Clearly these absorption lines indicate there are calcium atoms in our partition of intergalactic space while there are hydrogen atoms in the intergalactic space beyond the Local Group partition.
This partition (of matter and plasma) apparently surrounds a galaxy cluster.
This partition could be called an intracluster medium (ICM), as distinct from the pervasive intergalactic medium (IGM).
Chandra has observed evidence for an ICM in other clusters.
Some distant galaxy clusters show much of the ICM active in X-ray. This X-ray emission is from the ICM not an object in it.
Hydrogen has the Lyman Limit. If a hydrogen atom absorbs a frequency greater than this limit, the proton and electron separate. In an active galaxy cluster with radiation in the high ultraviolet to X-ray, much of the neutral hydrogen can become plasma of loose protons and electrons. Plasma in motion diverted by a magnetic field results in synchrotron radiation. The velocity of the particles determines the peak frequency so X-rays can result from fast moving plasma in the ICM, while the lower end is in the radio frequency range.
The hydrogen atoms in intergalactic space have a peculiar behavior with light.
The hydrogen absorption line accumulates a red shift by each hydrogen atom in the path. This is item (h) above.
Variations in the density of hydrogen atoms results in variations in a calculated distance which assumed a uniform density.
This is the reason for the Virgo Centric Flow where hydrogen absorption line red shifts vary in the direction of the Virgo constellation; visually closer galaxies have higher red shifts than more distant galaxies. This discrepancy is called a peculiar velocity.
The observation of item (h) should have prevented anyone assuming a red shift is from a velocity.
Unfortunately astronomers made that mistake so they assume a galaxy velocity increases by distance, an illogical conclusion.
Studies of galaxies with red shifts greater than 9, including the highest red shift at z=11.1, are still using the hydrogen absorption line.
A University of Oregon lecture published spectra of 5 galaxies with red shifted calcium absorption lines.
Therefore there are other galaxy clusters sharing the same characteristic as our Local Group; these clusters also have calcium atoms in their intergalactic space, or their ICM.
The lecture did not identify the galaxies in the sample so no galaxy cluster is identified.
The calcium red shifts among these 5 had z range from 0.004 to 0.2033.
a) much of the intergalactic space has a low density of hydrogen atoms.
b) there are some galaxy clusters with some calcium atoms in their ICM .
It is crucial to recognize these atoms in the intergalactic space can never indicate a motion of the galaxy whose light is passing through intergalactic space.
Galaxies are not really moving at multiples of the velocity of light as astronomers conclude from an absorption line.
That mistake resulted in the subsequent mistake of an an expanding fabric of space carrying these superluminal galaxies. This proposed fabric could have proposed limits, unlike the infinite space of the universe. There is no fabric of space.
The wrong interpretation of light passing through intergalactic space with a low density of hydrogen and calcium atoms has resulted in other notable mistakes.
M31 is wrongly assumed on a collision course with the Milky Way but this prediction cannot be justified using just a calcium absorption line from intergalactic space (i.e., the Local Group ICM).
Attached is a collection of images from Chandra of a galaxy cluster in wavelengths from radio to X-ray.
The X-ray and radio emissions are diffused, not from point sources.
The description of this cluster mentions hot gas. When gas atoms become hot they ionize so hot gas always means plasma which is matter with an electric charge.
The 2 statements:
1) "The hot gas that pervades clusters like Ophiuchus gives off much of its light as X-rays."
2) "Radio emission from electrons accelerated to almost the speed of light"
are both explicit observations of synchrotron radiation in the cluster ICM from its plasma.