A new scientific hypothesis is proposing something quite radical: Instead of a supermassive black hole at the center of the Milky Way, there is a large core of dark matter. This could explain some peculiarities in the current observations of the motion of stars around the core of our galaxy.
The central object in our galaxy is known as Sagittarius A* (pronounced A-star) or Sgr A* for short. It is estimated to be a supermassive black hole weighing about 4.15 million times the mass of our Sun. Evidence for this comes from the motion of stars around the center of the galaxy, in particular a star called S2, which has been used to test Einstein’s general relativity. The motion of this star and the change in color at the closest point to Sgr A* support the idea an extremely massive and compact object like a black hole lies at the core of the Milky Way. As the star goes around the black hole, its orbit shifts, and its closest point to the black hole changes with each turn. General relativity predicts exactly how much the orbit changes.
An unusual class of objects known as G class also orbit the galactic center, and calling them peculiar doesn’t do them justice. They are compact star-like objects but when they are at their closest point around Sgr A* they look like a stretched-out interstellar cloud. They look like gas but act like stars. However, there is some peculiarity to these objects' motion that is not perfectly explained yet, which allows for some freedom in considering alternatives to a black hole.
The new hypothesis, accepted for publication in MNRAS Letters and available to read as a preprint, has an idea as to what that may be. The team suggests the compact object is not a black hole but a dense core of a particular type of dark matter called a “darkinos”. The team claims the motion of S2 and the peculiarity of G2, one of the first discovered G class objects, are a better fit for their dark matter core scenario than a supermassive black hole. To test it further they incorporated the motion of the other stars in the S class and found they fit with their model.
Dark matter is a hypothetical form of matter that is expected to outweigh regular matter (like me, you, stars, and capybaras) in the universe 5-to-1. The existence of dark matter explains the rotation and motion of galaxies we have observed but experimental evidence of its existence is lacking. There are many dark matter candidates out there though, and darkinos are among them.
The name dark matter is a bit of a misnomer. It does not interact with light and it is an unknown so it was called "dark", but it is actually invisible matter. It interacts gravitationally and in some dark matter scenarios it can end up in "overdensities" of dark matter, or dark matter cores. The researchers believe that this could be the case at the center of the Milky Way.
Based on the assumption that Sgr A* is a big clump of darkinos, the team estimates that these hypothetical particles must have a mass of about one-ninth of an electron, and that this compact dark matter object is about 500,000 solar masses lighter than Sgr A* if it's a black hole.
Testing this hypothesis to either poke holes in it or find further evidence of a supermassive black hole might come sooner rather than later. When the Event Horizon Telescope took the first image of a supermassive black hole, M87*, in 2017 it also observed Sgr A*. Observations of the latter are yet to be published but are bound to offer up interesting information about our galactic center, and of course, new observations are already in the works.