New discovery bring us closer to understanding the mysterious dark matter in the Universe

It is everywhere in our Universe: dark matter is thought to make up the vast majority of all matter in the Universe. We know it is there, but we still know very little about what it actually is. That is why it is called ‘dark matter’.

Now, the first discovery of a stream of stars, known as a globular cluster stream, in a galaxy beyond the Milky Way could shed new light on the mystery of this dark matter.

Such streams are a well-known phenomenon in the Milky Way, where several have been identified. But this is the first time a stellar stream originating from a globular cluster has been discovered beyond our own galaxy. The finding has now been published in the scientific journal Nature .

”The discovery opens up new opportunities to investigate dark matter, which is one of the greatest unsolved mysteries in modern astrophysics,” says Sarah Pearson, who was part of the research team behind the discovery. She is a co-author of the Nature paper together with PhD student Julie Kiel Holm, who is the first author.

The study was carried out at the Niels Bohr Institute at the University of Copenhagen, where Sarah Pearson worked before recently joining DTU Space as a researcher and associate professor. She is also Julie Kiel Holm’s PhD supervisor.

Stellar streams as dark matter detectors

Globular cluster stellar streams can be used as a kind of ‘dark matter detector’. This is currently a highly active field of research in astrophysics, with Danish researchers at the forefront.

The field has advanced significantly as ESA’s Gaia satellite, together with ground-based telescopes, has made it possible to discover many more of these stellar streams in the Milky Way. In recent years, large new datasets and advanced analysis methods have turned globular cluster streams into one of the most promising tools for studying galaxies.

The stellar streams form when the gravitational pull of a galaxy slowly tears apart a dense collection of stars known as a globular cluster, stretching its stars out into a long, narrow stream.

They are particularly useful for studying dark matter because they are typically found in the outer regions of galaxies. Out there, there is much less cosmic clutter, such as gas and large concentrations of stars, to complicate the data picture.

If researchers detect an unexplained gap or disturbance in an otherwise smooth stellar stream, one possible explanation is an invisible concentration of dark matter that has passed through or close to the stream. By analysing these precise cosmic signatures, astrophysicists can learn more about the mass and location of the invisible structures formed by dark matter.

However, globular cluster streams have been extremely difficult to detect beyond our own galaxy because their signals are so faint and can be hidden by signal noise from other phenomena. In this case, the stellar stream was discovered in an so-called ultra-diffuse galaxy named UGC9050-Dw1.

”We have observed a globular cluster stream in an ultra-diffuse galaxy. The fact that we are seeing this outside our own galaxy, the Milky Way, makes it a major and unique discovery, because it has never been done before,” says Julie Kiel Holm.

So researchers now have a familiar tool that can be applied to galaxies other than the Milky Way. This will allow them to make new measurements beyond our own galaxy, both testing what we already know and providing new insights into dark matter.

95 percent of the universe’s mass-energy content is unknown

The vast majority of matter in the Universe is dark matter. Yet we still do not know what dark matter is actually made of.

Looking at the total mass-energy content of the Universe, ordinary matter, from stars and planets to gas and dust, accounts for only around 5 per cent. The 95 percent is almost unknown. Dark matter makes up around 27 per cent, while the remaining approximately 68 per cent is dark energy, another phenomenon that scientists are still trying to understand.

”The discovery opens up entirely new possibilities. We can search for these globular cluster streams in galaxies other than the Milky Way. In the longer term, we may also be able to measure how much dark matter is present in other ultra-diffuse galaxies,” says Sarah Pearson.

Beyond the discovery itself, the researchers demonstrate for the first time that globular cluster streams can be used as a tool to measure dark matter in galaxies beyond the Milky Way. This is important ecause dark matter plays a crucial role in the Universe but remains so poorly understood.

“We show that a well-established tool from our own galaxy can be used to understand other galaxies, where measuring the distribution of dark matter has otherwise been very difficult,” says Julie Kiel Holm.

Most current knowledge is based on Milky Way observations

In the study, the researchers also estimate how much mass, and therefore dark matter, the UGC9050-Dw1-galaxy contains and how that mass is distributed.

According to Julie Kiel Holm, the discovery can help researchers investigate whether our current picture of dark matter, which is based in very much on observations of the Milky Way, also applies to other galaxies.

”Our current understanding of dark matter is largely based on a single galaxy, our own. With a larger dataset, researchers can begin to investigate whether the same models apply universally, or whether dark matter behaves differently depending on its environment,” she says.

With new space telescopes and missions such as NASA’s Nancy Grace Roman Space Telescope and ESA’s Euclid, the researchers expect the number of observable globular cluster streams to increase significantly.

”This is an incredibly exciting field of research that is developing rapidly,” states Sarah Pearson.

”It gives us entirely new opportunities to gain insight into what dark matter is, and therefore what most of the matter in the Universe is made of”.