Dark matter doesn’t interact with light and it’s invisible to our instruments. But it is believed to interact with gravity, so researchers have used this fact to cleverly produce the first observation of a dark matter "bridge" that connects galaxies.
A team from the University of Waterloo has used subtle distortions in the shapes of distant galaxies to “see” the filaments of dark matter stretched between galaxies. Dark matter filaments are believed to connect galaxies and galaxy clusters forming the so-called cosmic web.
The study, published in the Monthly Notices of the Royal Astronomical Society, combined the distortions seen in 23,000 galaxy pairs to build up a composite image, or map, of the dark matter bridge. The paper also suggests that these structures are stronger when they are less than 40 million light-years apart.
“For decades, researchers have been predicting the existence of dark matter filaments between galaxies that act like a web-like superstructure connecting galaxies together,” co-author Professor Mike Hudson said in a statement. “This image moves us beyond predictions to something we can see and measure.”
Matter bends space-time and although dark matter is not dense like a black hole, the effect can be picked up by our instruments.
The team used observations from a multi-year sky survey at the Canada-France-Hawaii Telescope. The looked for weak gravitational lensing in galaxies 4.5 billion light-years away. Gravitational lensing is usually shown as dramatic warping of the object's shape (that’s strong gravitational lensing) but it can be more subtle.
In weak gravitational lensing, the shape of galaxies change slightly and astronomers work it out by looking at how warped the neighbors of those galaxies are. If they all seem to be stretched in the same direction than some unknown mass must be bending space-time. In this case, the unknown mass is believed to be dark matter filaments.
“By using this technique, we’re not only able to see that these dark matter filaments in the universe exist, we’re able to see the extent to which these filaments connect galaxies together,” added co-author Seth Epps.
The presence of this network of bridges is an important prediction of the standard cosmological model that requires the existence of dark matter and dark energy, which have not been proven to exist yet.
False color map of dark matter filaments between galaxies. The two galaxies are shown in white and the dark matter in red. S. Epps & M. Hudson / University of Waterloo