The universe's dark matter does not just exist on the outskirts of galaxies, it also forms bridges between them. Now for the first time, a map has been created showing the connections between thousands of nearby galaxies, including our own.
Dark matter, by its nature, is hard to map. Its existence was discovered because the rate of spin of galaxies proved more mass was present than we could see. However, despite likely making up approximately 85 percent of the matter in the universe, it does not absorb, reflect or interact with light, hence "dark". Locating material we can't visualize, and whose nature we don't even know, has remained a challenge.
Despite this, four years ago astronomers created the first map of a bridge made of filaments of dark matter between galaxies. They detected its presence by looking for distortions in the visible matter of the galaxies at either end, caused by the gravitational pull of the bridge.
Previous dark matter maps have involved galaxies immense distances away. "Ironically, it's easier to study the distribution of dark matter much further away because it reflects the very distant past, which is much less complex," Penn State's Dr Donghui Jeong said in a statement. "Over time, as the large-scale structure of the universe has grown, the complexity of the universe has increased, so it is inherently harder to make measurements about dark matter locally."
Jeong and colleagues simulated the distribution of galaxies using a convolutional-neural-network-based deep-learning algorithm and compared its output with data on 17,000 galaxies within 650 million light-years of our own Milky Way. Combining the two sources of information they published a map of our local dark web in a paper accepted for publication in the Astrophysical Journal.
Without being able to see the dark matter, or conduct any direct measurements, the accuracy of the map cannot be definitively confirmed. However, if their model is good, it would be expected to reproduce certain features we can see. Although Jeong admits the match is not perfect, it's close enough to suggest the simulations are on the right track.
For example, a structure known as the “Local Sheet”, which connects our Local Group of galaxies to the much larger Virgo Cluster and the famous M81 and other galaxy groups, can be seen. So can the “Local Void”, which is exactly what the name suggests. These features were known, but the paper also claims a previously undetected connection between the Local Sheet and the Fornax Wall.
"Having a local map of the cosmic web opens up a new chapter of cosmological study," Jeong said. "We can study how the distribution of dark matter relates to other emission data, which will help us understand the nature of dark matter. And we can study these filamentary structures directly, these hidden bridges between galaxies."
This includes dark matter bridges between the Milky Way and other members of the Local Group, whose strength and distribution will determine our galaxy's future evolution.
Newly discovered galaxies, faint enough to have been overlooked until now, will help improve the accuracy of future versions of the map. Between the (fingers crossed) upcoming JWST and the next generation of giant Earth-based telescopes now under construction, we should soon get plenty of those. Nevertheless, plenty of uncertainty will remain in the areas hidden behind the disk of the Milky Way, which hides our view of everything else.