I suspect that the “black” matter in question is the phenomenon known in physics as “dark” matter (darkmatter – Wikipedia).
This is “invisible” (i.e. non-electromagnetic spectrum) matter, which is only noticeable by its gravity interaction in the observation of galaxies.Thus, the rotational speeds of spiral galaxies are often greater than can be explained by luminous matter alone, or the distribution of luminous matter in a galaxy requires the presence of further invisible matter. As can be read in the Wikipedia article, the proportion of this invisible matter is 63% of the matter that we can notice through its gravity effect.
Exactly what this dark matter is is still unknown.While there are conjectures and postulated particles that could be considered, it has not yet been possible to prove any of the theories or new particle types:
- Baryonic (i.e. the same variety as the atoms in our environment) dark matter
- Cold (non-luminous) gases or dusts
- MACHO – Wikipedia (Massive, astrophysical, compact halo objects)
- Non-Baryonic Dark Matter
There are theories that postulate the so-called Neutralino – Wikipedia
In addition to the attempt to explain the deviation between observed behavior of galaxies and the observed glowing matter, there is also an attempt to explain these deviations by a changed form of the gravitational law, such as the “MOND” Modified Newtonian Dynamics – Wikipedia, Tensor Vector Scalar Gravitational Theory – Wikipedia, Scalar-Tensor-Vector-Gravitational Theory – Wikipedia.
But as I said, to date there is no positive evidence that any of these theories are true.