Despite plenty of circumstantial evidence for the existence of dark matter – the mysterious form of matter that dominates galaxies and clusters – astronomers have yet to make direct observations.
But the search is not over. A hypothesis about the nature of black matter is that some of them might be self-interacting, which means that the individual particles slightly interact with each other. If true, there would be a host of subtle observational clues for the existence of this subclass of dark matter.
A few of these clues were recently described in a paper submitted for publication in the journal Reviews of Modern Physics and published in the Preprint Database. arXiv (opens in a new tab).
Related: This is how colossal clusters of galaxies are revealing the secrets of dark matter
Strong gravitational lens
Strong lensing occurs when there is a happy coincidence of observations. When astronomers look at a distant galaxy cluster, for example, they may also see light from even more distant galaxies passing through the cluster. The mass of the galaxy cluster (typically 10^14 or 10^15 times the mass of the sun) is so large that it bends and distorts the fabric of space around it. This distorts background images galaxiestransforming them from familiar windmills and elliptical structures into long wavy snakes and other fun shapes.
Astronomers can reconstruct these distorted images and use this reconstruction to determine how much mass is in a cluster and where it is clumped together. Typically, self-interacting dark matter has a different “aggregation” than ordinary non-interacting dark matter. Dark matter without interaction will continue to accumulate to incredibly high densities in the nuclei of galaxy clusters, because there is nothing else to stop it. But when dark matter interacts with itself, it slows down the core building process and smooths things out in the inner parts of a cluster.
Detailed observations (such as those recently provided by the James Webb Space Telescope) of the mass distribution inside galaxy clusters could provide a clue to the existence of dark matter.
Weak gravitational lens
Unlike strong gravitational lens, a weak lens does not require a massive obstruction. Instead, as light from many distant galaxies makes its way through the cosmos, the accumulation gravity of all the galaxies and other objects that the light passes near during its journey modifies it in a minute way. For example, galaxies in a particular direction may appear somewhat rounder or larger than galaxies in other directions.
Strong gravitational lensing requires lucky alignments, so we don’t have many clusters to work with. But even if a weak gravitational lens produces a much weaker effect, we have a lot more data to use. Astronomers are very excited for the launch of the Nancy Grace Roman space telescopewhich will provide detailed low-lens maps of the nearby universe and could tell us if dark matter is interacting with itself.
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In the 1970s, the astronomer Vera Rubinobservations of the movement of stars inside galaxies provided the first major evidence for the existence of dark matter. In short, the galaxies rotate much too quickly. If we add up all the mass of a galaxy based on what we can see, there just isn’t enough gravity to hold stars with those kind of fast orbits. Therefore, there must be more mass than we can see: dark matter.
Again, because self-interacting dark matter clumps together differently than non-interacting matter, this can alter the spin curves (plots of the velocities of stars in different orbits) of galaxies.
Deformation of galaxies
Throughout their lifetime of billions of years, matter constantly rains down on each galaxy from its surroundings. In other words, each galaxy swims in an ocean of stuff. This material can include both ordinary matter and dark matter. When dark matter interacts with itself, it causes the dark matter portion of a galaxy to lag slightly behind normal matter (because normal matter can swim through all surrounding things without issue).
This can cause galaxies to have two slightly offset nuclei: one made of ordinary matter and the other made of dark matter. This shift triggers tidal disturbances throughout the galaxy, potentially even causing the galaxy’s disk to warp. Future detailed observations of the galaxies may reveal disk warping that only interacting dark matter can explain.
When giant clusters of galaxies merge, astronomers can peer down at the wreckage to figure out what’s inside. For example, the famous bullet cluster shows what happened when two clusters merged: stars and dark matter (measured by gravitational lensing) passed through each other intact, while all the free gas in the clusters collided at the center of the collision. .
The fact that dark matter is on the periphery of the system tells us that dark matter does not often interact with itself; otherwise it would have become tangled in the center next to the gas. The Bullet Cluster and other similar clusters allow astronomers to place limits on how strongly dark matter can interact with itself. More observations will lead to more precise boundaries and possibly even positive evidence of self-interacting dark matter, if that provides a better fit for the observations.
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