DARK MATTER SEEMS REAL... BUT IS IT DARK?

Studies show that the motion of large scale objects (galaxies and clusters of galaxies) do not agree with the predictions of ordinary gravitational laws acting on ordinary objects.

The most common explanations propose either 4 times as much "dark matter" as ordinary matter, or changes in the laws of gravity.

Recent observations favor non-interacting matter as the source of the effect, rather than changes in the laws of gravity. Observations using the gravitational lens effect show two colliding galaxy clusters in which ordinary matter in the two clusters interact, passing through each other more slowly than some different matter (identified with the "dark matter"), which passes through unimpeded by collisions.

(One of the effects ascribed to "dark matter" is that gravitation clumps it, and then the concentrations of clumped dark matter help clump ordinary matter through gravitational attraction, assisting in the formation of galaxies, stars, etc. The clumping however must be on a huge scale. I am not aware of any observations of non-uniformity of the dark matter on a distance scale small enough to affect anything in our solar system.)

Note, by the way, that as a matter of terminology, because "dark matter" does not interact with itself or with ordinary matter, it seems more appropriate to call it transparent matter. "Dark" usually implies an ability to absorb light, while "transparent" means that light passes through the object, and "dark matter" does not absorb light (or interact with anything else, except gravity), so it is "transparent," not "dark." "Invisible" matter is not quite right either, since we can "see" dark matter from its gravitational effects.

"Transparent matter" is not nearly as catchy a phrase as "dark matter," but it may be more accurate.

Here are some references to the recent observations and other information about (transparent) "dark matter."

Wikipedia
NASA News
Technical paper