Astronomers haven't made direct observations of dark matter despite lots of circumstantial evidence.
The search is still going on. One hypothesis for the nature of dark matter is that some particles interact with each other. There would be a host of clues if this is true.
A few of these hints were recently outlined in a paper submitted for publication in the journal Reviews of Modern Physics and published to the pre print database arXiv.
Here is how dark matter secrets are revealed.
There is a chance that there is a coincidence. Astronomers can see light from distant galaxies when looking at a distant cluster. The fabric of space around it is warped by the mass of the galaxy cluster, which is usually 1014 or 1015 times the mass of the sun. The distorted images turn the pinwheel and elliptical structures into fun shapes.
Astronomers can use that reconstruction to figure out how much mass is in a cluster and where it is clumped up. There is a difference between self-Interacting dark matter and regular, non-Inter acting dark matter. There's nothing else that can stop non-interacting dark matter from piling up in the cores of galaxy clusters. The process of core building is slowed when dark matter interacts with itself.
A clue to the existence of dark matter could be provided by the distribution of mass inside galaxy clusters.
Weak lensing doesn't need a huge obstruction. The accumulated gravity of all the galaxies and other objects that the light passes near changes in tiny ways as it travels through the universe. The galaxies in one direction may be a bit rounder than the other direction.
We don't have a lot of clusters to work with because of the strength of the lens. We have a lot more data to use despite the small effect of weak gravitational lensing. The Nancy Grace Roman space telescope will provide detailed weak-lensing maps of the nearby universe and possibly tell us if dark matter is acting alone.
The first evidence for the existence of dark matter was provided by astronomer Vera Rubin in the 70s. They are spinning too fast. There isn't enough gravity to hold on to stars with fast circles if we add up all the mass in a galaxy. Dark matter has to be more mass than we can see.
The rotation curves of the galaxies can be changed by this because self-Interacting dark matter clumps up differently than non-Inter acting dark matter.
Every galaxy has a constant rain of material from its surroundings throughout their lifetimes. The universe is in an ocean of stuff. Regular matter and dark matter can be included in this material. The normal matter can swim through all the surrounding stuff without a problem when dark matter interacts with itself.
There are two slightly offset cores, one made of regular matter and one made of dark matter. The offset could cause the disk of the galaxy to warp. Future detailed observations of the universe may show a warp in the disk.
Astronomers can see what's inside giant clusters of galaxies when they collide. The Bullet Cluster shows the stars and dark matter passing through each other without being hit by the other in the center of the collision.
The dark matter is on the outskirts of the system, which means that it doesn't interact with itself often. Astronomers can place limits on how strongly dark matter can interact with itself by clustering. If there is a better fit for observations, more observations will lead to more precise limits.
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