Neutron stars are collapsed stars that have used up all of their fusion fuel. Typically what happens at that point in a star’s life is that they collapse, but the extent of the collapse is largely a factor of how much mass they had. A star the size of our sun will collapse into a white dwarf (dense but still composed of atoms with electron clouds), but a heavier star will often collapse into a much denser neutron star (the gravity has crushed the electron clouds out of existence with only neutrons left, at least at their core). Heavier stars yet will collapse into black holes (where the gravity overwhelms all repulsive forces between particles and causes the whole structure to collapse into an infinitely dense point, a singularity).
Apparently, the problem is that there aren’t as many neutron stars at the center of the galaxy as there should be according to astrophysical predictions. One possible explanation is that, over time, heavy neutron stars attract too much dark matter into their cores, and that the additional mass collapses them into black holes. It’s an interesting theory, but as the article describes, it’s just one of many possibilities.
But reading this article made me wonder how much of the mass of the super-massive black hole at the center our galaxy might be composed of dark matter. Or if it’s possible for dark matter in other regions to collapse into a black hole without ever going through the star stage. It seems like it would depend on to what extent dark matter interacts with itself.
And that gets the the problem with any theory involving dark matter. We just don’t know what it is yet. Dark matter is only detected by its gravitational effects. No one has yet managed to detect it in any other way. There are lots of ongoing experiments to do just that. Hopefully one of them will eventually make that detection, and the nature of it will tell us more about what appears to make up the majority of the matter in the universe.