There may be a “dark matter nucleus” at the center of the Milky Way instead of a black hole

Scientists have long believed that the object at the center of our galaxy is a super-sized black hole called Sagittarius A* (Sagittarius A*). A recent study raises a new possibility that this giant object is not a black hole, but a cloud of very dense dark matter.

The mass of Sagittarius A* is estimated to be about 4.15 million times that of the Sun. For an S-class star S2 orbiting it and its characteristics, according to the hypothesis that Sagittarius A* is a black hole, it can be explained by Einstein’s general relativity, so for many years, scientists have believed that Sagittarius A* is a giant black hole.

However, the universe is never that simple. Scientists have discovered another unusual type of object that is also spinning around the center of the Milky Way galaxy, called a G class object. Scientists have found that the rotation of these objects around the center of the Milky Way is somewhat biased when explained by relativity.

Normally this type of object is compact and behaves like a star, but when it orbits closest to Sagittarius A*, it is elongated to become like a nebula. In other words, a G-type object is a special kind of object between a nebula and a star.

The new study, published on May 17 in the preprint arXiv, suggests that Sagittarius A* may be a dark matter nucleus called darkinos. According to this hypothesis, it can better explain the orbits of S-class stars as well as G-class objects orbiting around it.

Dark matter is also the kind of matter that scientists currently hypothesize exists to make up the universe; it does not interact with light, only with gravity, and scientists believe it is as much as five times more abundant in the universe than those detectable substances. With the introduction of the concept of dark matter, scientists can explain the trajectories of galaxies, the distribution of positions that cannot be explained by gravity, and other issues. Scientists do not yet know exactly what dark matter is, and there are many different models of dark matter under this big concept, darkinos being one of them.

The hypothesis is that dark matter, when interacting with gravity, sometimes becomes a dense mass of dark matter, forming a “dark matter nucleus” like form. The study suggests that the massive object at the center of our galaxy may be darkinos, rather than the super black hole it has been thought to be.

The team constructed a new model of Sagittarius A* as a dark matter nucleus, testing it first with the orbits of S2 and G2 (one of the first G-like objects discovered). They were both found to be better explained. Several other S-class stars were then further introduced and their kinematic characteristics were found to be consistent with the predictions of this model as well.

The researchers estimate that the particles that make up this dark matter have only one-ninth the mass of an electron, so overall, the mass of this dark matter nucleus is about 500,000 solar masses lighter than if it were a super black hole.

Researchers say any theory needs evidence to back it up. Close observation of Sagittarius A* has begun since the Event Horizon Telescope took the first pictures of the black hole, M87*, in 2017.

This paper is forthcoming in MNRAS, a peer-reviewed academic journal in the field of astronomy and astrophysics in the UK.