According to the mainstream explanation, the dark matter is one hypothetical particle or another that interacts with itself either very weakly or not all, and that does not have any way of dumping its energy to other physical entities to form aggregates, and that bizarrely seems to interact with matter only gravitationally (weak interaction is not totally ruled out).
There is no stronger indication of a crisis of historic proportions in physics today than the twin problems of dark matter and dark energy. In an effort to "explain away" the increasing number of associated problematic observations, mainstream cosmologists and astrophysicists are digging their heels either in the mire of the so-called Lambda-CDM model of the universe or in proposing yet more "aethereal" explanations. In my opinion, these problems have become the "aether problem" of our age.
Here is the latest example of this game: The common plane of orbit of satellite galaxies around their host galaxy is unexplainable with the Lambda-CDM model. (Please note that such a plane orbit is expected in my rotary spacetime model of the universe.)
One recent paper proposes the existence of some "invisible walls" in the universe to explain it: