Universe has more hydrogen than we thought

Re: Sir

"If they can't detect dark matter, how do they make those 'dark matter maps' that look like the honeycombe understructure of the Universe?"

Simplistically, by measuring where the visible (non-dark) matter is.

The existence of "Dark Matter" was deduced from measuring the speed of rotation of galaxies and their mass.

Speed of rotation is measured by looking at the shift in the spectrum of the visible light emitted from a galaxy. Assuming a galaxy is 'edge on' relative to us, one side will be moving towards us, and thus the spectrum is shifted towards the blue end, and the opposite side moving away from us, and shifted towards the red. By measuring the relative difference in shift between the two sides, it is possible to work out how fast the galaxy is rotating.

Mass is estimated from observations of what we can see - visible light for the stars, comparing visible and infrared observations to identify the dust (blocks the former but not the latter), radio observations for hydrogen, etc.

Turns out that galaxies rotate too fast compared to the gravitational force generated by the matter we can observe. If the only matter they contain is what we can see, they would fly apart, flinging their starts, dust and gas off in all directions.

The fact that they don't leads to one of two conclusions:

1. Our current theories of gravity are incorrect. Serious consideration has been given to the idea that gravity may act differently over long distances than short ones. This is not currently a theory that holds much sway.

2. That the universe contains a whole lot more mass than we have been able to observe, i.e. "Dark Matter". The trouble is that weird/exotic types of unknown particle or other bogeyman make better headlines than forgetting to look under the bed for the missing mass.

To be fair, lots of 'boring' things like undetected gas or shedloads of brown dwarfs or similar objects that are to dim to detect have and are being pursued, as this story proves. The problem is that the discrepancy between what we can observe and the missing mass is so large that it seems entirely reasonable that there is a whole class of 'stuff' out there that we simply have no idea about and thus no (current) means of detecting.

Anyway, back to the original question. The large scale structure of the universe has been mapped (in a few directions) by measuring the distribution of galaxies. These do form a sort of 'honeycomb' structure with filaments and walls and great gulfs of empty space between them. The Dark Matter that holds galaxies together is also thought to be responsible for the formation of these larger scale structures.

Basically, current theories suggest that the dark matter is in the same places as the stuff we can already see because the latter is held together by the gravitational force generated by the former. As with all scientific theories, something may come up in the future that proves we were barking up completely the wrong tree all along.