Galaxies come in all shapes and sizes, but the essential ingredients seem pretty consistent. There’s usually a massive black hole at the center, a group of stars and gas, and a generous amount of dark matter to help glue everything together. While dark matter is actually dark, there is a swirling core of heated matter that shimmers with the beauty of stars, gas, and a night city. However, a recently discovered dwarf galaxy located just 94 million light-years away does not live up to expectations. It is designated FAST J0139+4328 and does not emit optical light. In fact, it emits almost no light.

FAST J0139+4328 looks like a dark galaxy. With the exception of a small number of stars, the galaxy appears to be almost entirely composed of dark matter. The article describing the discovery has been accepted for publication. Astrophysical Journal Letters and available on the arXiv preprint server.

“These findings provide observational evidence that FAST J0139+4328 is an isolated dark dwarf galaxy,” wrote a team of astronomers led by Jin-Long Xu of the Chinese Academy of Sciences in Beijing. “This is the first time an isolated dark galaxy has been detected in the near universe.”

Dark matter is the main explanation for the strange discrepancy between the amount of normal or baryonic matter observed in the corners of the universe and the gravitational force required to hold it together. Simply put, there isn’t enough baryonic matter to explain all of gravity. Galaxies are spinning so fast that they have to break apart with nothing else tying them together.

Whatever is responsible for this extra gravity is elusive. It doesn’t seem to interact with ordinary matter in any way other than gravity; Nor does it emit any form of radiation that we can currently detect. We cannot see the source of this extra mass. However, making room for some unknown material goes a long way toward solving the problems we’ve seen.

But the dark matter theory isn’t perfect either. One problem is the discrepancy between simulations of the distribution of dark matter in the universe and the number of dwarf galaxies orbiting large galaxies. There are far fewer dwarf galaxies than the simulation predicted. This is known as the dwarf galaxy problem.

It is possible that we are unable to detect some types of dwarf galaxies, such as those with very few stars, mostly composed of gas and dark matter. Finding enough will help eliminate the entire scarcity of dwarf galaxies. Some dark candidate galaxies have been detected, but their proximity to other structures makes them difficult to distinguish from debris clusters that have been torn apart by stronger gravitational forces.

An ideal dark galaxy candidate would self-drift in isolation through space where its identity is impossible to misunderstand.

FAST position of J0139+4328 in (a) optics, (b) near infrared, (c) near ultraviolet, and (d) mid infrared. The yellow circles in a and b are the foreground stars. (Xu et al., arXiv, 2023)

Xu and his colleagues used the Aperture Spherical Radio Telescope (FAST) in China to search for such a galaxy. They used the telescope as a “fill-in” project during breaks in the observing program to search for radio emission from large clouds of neutral atomic hydrogen (HI) in intergalactic space looking for features consistent with galaxies.

And they succeeded: The radio waves emitted by the HI cloud 94 million light-years away were consistent with a spinning disk galaxy without the optical light expected of it. Other observations in infrared and ultraviolet radiation revealed a weak scattering of stars. Together, these data allowed the researchers to identify the features of the galaxy, which they named FAST J0139+4328.

According to the team’s calculations, the galaxy’s stellar upper limit contains 690,000 solar masses and 83 million solar masses of HI. The total baryonic mass of the galaxy is about 110 million solar masses. But that’s just a drop in the bucket of the galaxy’s total mass. The team was able to calculate the rotation speed of FAST J0139+4328 and based on that, its total mass of 5.1 billion solar masses. This means that about 98 percent of the galaxy is dark matter.