Much of the visible matter in the universe is difficult to detect

Recommendable! Perhaps only fraction of all visible matter is actually visible from earth.

"... Some matter, like dust and gas at the very edges of a galaxy or galaxy cluster, doesn't emit any light of its own, and very little light shines on it. By contrast, other matter is too close to light sources to be seen. The light from matter that sits close to the heart of galaxies and galaxy clusters is hidden behind the brighter light of other objects like stars.

The result is that only a small percentage of what’s considered visible matter is actually visible. “Ordinary matter is only 5% of the total energy in the universe,” Schaan says, “and if you look at stars within galaxies, you're only seeing 5 to 10% of the ordinary matter.” ... Either way, physicists have concluded that around 16% of the matter in the universe is ordinary, while the rest is dark matter. ...

To map out the rest of the ordinary matter, then ... teams, have taken to looking for shadows. Using the CMB as a backlight, they look for a kind of galactic watermark whereby the heat and motion of dust and gas at the edges of galaxies boost the energy of the CMB light. These effects, known collectively as the Sunyaev-Zeldovich effects, can reveal both where the dust and gas are and also their speed and energy—thus providing essential new details on how it got there. ...
An alternative ... is to look instead for X-rays emitted by baryonic matter. Although galactic processes may blow some of that matter away, a considerable amount remains—and it gets very hot. In large galaxy clusters ... gas temperatures can reach into tens of millions of degrees, at which point the gas emits a lot of X-rays. ..."

Searching for the matter that hides its shine | symmetry magazine Just because matter is visible doesn’t mean it’s easy to see.