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They don't detect the gravity per se.

There's a certain amount of deduction going on here. Theory (meaning computer simulations) tells them that there should be self-gravitating streams of hot gas between the galaxies.

You can get a taste of what these streams look like and how they form in a couple of simulations I ran using the National Scalable Cluster Project supercomputers here at Penn, using parallel code written by Prof. Paul Bode.

This animated .gif shows the mass density of a simulated portion of the universe. Black is lowest density, followed by blue, green yellow, red, and finally white as highest density. Later images show later times, when more structure forms due to gravitational attraction. As the movie plays, the universe ages from 50 million to 12.5 billion years. During this time, our simulation volume actually expands by a factor of 40, but this expansion isn't shown.
Here is another such universe, showing more resolution (WARNING: 7 MB).
Here is the first frame of that universe, and here is the last frame.

Since they expect the streams to be there, they set about to detect them by looking at the spectra of distant quasars. Since there are so many streams, they reasoned that some quasars must end up behind streams, from our point of view. In that case, the streams will block out part of the light from the obstructed quasars in a characteristic way in the x-ray band. This obstruction is what they have detected.

Note that this doesn't allow them to image the streams themselves; they just see the effect of the streams at a small number of points on the sky. They call them streams because that's what they expected to see. The same data might support the hypothesis that there are big gasseous blobs out there. But either way, the total mass of the gas can be estimated, and it is non-negligible.

Thank you, Physicist.

I count on you for no-BS facts.

If I fail to understand those facts, well that is my failing.

Any thoughts on how this "hot gas" got to be so hot, and why it hasn't radiated most of it's heat out into the surrounding Universe?

Also, is it thought that the "hot gas" is still being heated? If so, what's the mechanism and source of energy to heat it?

Thanx...

The latest Scientific American has a cover story along the lines of "Do We Really Need Dark Matter?" If it lies in rivers and pulls ordinary matter with it, does that mean we do? (The proposed alternate theory revises Newtonian dynamics so that less force is needed to produce very small accelerations.)