j-lee00 said:
why is the temperature distribution of hot gases inside galaxies evidence for dark matter
You will get a better, more focused answer if you provide a link to an article where dark matter and hot gas are discussed.
there are a lot of different kinds of evidence of dark matter
1. maps of dark matter concentration in clusters, made by gravitational lensing
2. the flatness of space---it wouldn't be so flat without the extra matter
3. rotation of galaxies---faster than it could be without the extra matter
4. displacement of dark matter concentration from ordinary gas, when clusters collide.
#4 could be what you have read about, and are asking about. I'm just guessing.
When two clusters collide, each brings with it both a blob of ordinary gas intergalactic medium and a blob of dark matter. The blobs of hot gas collide and can stop each other at the point of collision. While the galaxies themselves are widely enough dispersed that the two clusters can pass THROUGH each other. And the blobs of dark matter also are able to pass through each other to some extent and stay with the galaxies.
So what is observed after a collision is the hot gas blobs combine and are left behind at the collision site, and the galaxies PLUS their dark matter clouds separate---and the two clusters continue on their way.
And one can see the dark matter around the two clusters, again by gravitational lensing.
It is a way of being sure that the lensing is not being caused by the massive clouds of ordinary gas which accompany clusters. Because in this case the gas is cleared out of the clusters by collision.
I haven't given all the steps in the argument, but this is one case where mapping the hot gas by its Xray emission and then mapping the dark matter by lensing and comparing maps, helps to clarify the issues and give people confidence in their interpretation of the lensing data.
In sum, there are a lot of very different kinds of evidence and they help rule out alternative explanations (because with anyone single observation you can always think of an alternative explanation)
and why is Type 1a Supernovae evidence for dark energy
Again it might help if you gave a link. It would show what you already understand and don't need to have explained
The short story is that Ia supernovae are standard candles so their brightness or dimness indicates distance. And one can compare their distance with their redshift.
Our earlier comparison used closer stuff, like Cepheid standard candles, and we got an idea of the relation between distance and redshift based largely on more nearby stuff.
And based on that we expected distance and redshift to be related in a certain way to increase together, as we looked farther back in time.
But we found the 1a SNe were DIMMER than expected for a certain redshift, and so they were FARTHER than expected, so therefore the redshift is increasing more SLOWLY than expected, as you look farther back in time. So therefore the universe was not expanding as fast, back THEN, as we expected it would be, looking at things now.
So the conclusion, adjusting the model to fit the data, was that there has been a slight increase recently in the rate of expansion.
The time derivative a'(t) of the scalefactor, which according to our earlier model should have been decreasing with time, was instead very slightly increasing with time.
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Now, as with anything else, you can find alternative explanations. One would be that we are at the center of a void and the stuff at the edges is pulling and causing acceleration. But this doesn't work so well. The dark energy explanation has something else going for it besides the 1a SNe. It turns out that if you calculate the amount of dark energy needed to explain the acceleration, it comes to about 70 or 75 percent of what is needed to cause spatial flatness. And that is exactly what is needed to explain flatness. Because the ordinary and dark matter was estimated to cover only about 25 or 30 percent of what was needed.
So that clicked.
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No one piece of data points uniquely to a single explanation. It is always how the different pieces of evidence fit together.
