Abell 4038 is a cluster of galaxies located in the constellation Sculptor at a distance of roughly 415 million light years (127 Mpc) [1]. Although this cluster is named after George Abell, who spent endless hours staring at photographic plates so that he could identify thousands of these clusters, it appears that Abell 4038 might have been discovered by someone else. The earliest scientific paper mentioning the cluster that I could find was written by A. R. Klemola and was published in 1969 [2]. I suppose the cluster was eventually included in one of Abell's later catalogs, which is why it got the name Abell 4038.
The cluster (or at least the X-ray emission from the gas between the galaxies within the cluster) has a diameter of about half a degree [3], which is equivalent to the diameter of the Moon, and while I can't easily find any information on the number of galaxies within the cluster (which would be difficult to count anyway), the total mass estimated for the cluster is about 150 trillion times the mass of the Sun [4]. For context, the size of a spiral galaxy like the Milky Way will be around 100 billion times the mass of the Sun, so Abell 4038 has 1500 times that mass. However, some of the mass will be in the form of hot gas between the galaxies, and some of the mass will be in the form of dark matter, so Abell 4038 does not necessarily contain 1500 large spiral galaxies.
The most interesting thing about Abell 4038 is the extra radio emission unassociated with any specific galaxy that has been detected in some off-center locations in the cluster [5, 6, 7]. The images look a bit blurry, but the radio emission has been divided up into four different parts: an arc-like thing on the west side of the cluster, a thing called a bridge extending from the arc into the center of the galaxy, a loop-like thing at the south end of the arc, and a blob of radio emission in the northern part of the cluster, all of which are spread over an area of several hundred thousand light years.
These objects have collectively been labelled as a radio relic. They most likely originated from an active galactic nucleus (AGN) with a supermassive black hole within one of the galaxies in the cluster. At some point in the distant past, gas was falling into the black hole, but a lot of it ended up being deflected away from the black hole by its magnetic fields and flying away from the black hole's poles in narrow beams or jets of ionized gas that extended far outside the galaxy. However, in the present (or, to be more accurate, at the point in time at which we are seeing the cluster now), no gas is falling into that black hole, so no gas is getting deflected into jets above the poles of the black hole. All we are left with is the radio relic, which is ionized gas in the space in between the galaxies that is emitting radiowave emission. These radio relics are relatively rare, so finding one is actually quite notable.
However, that radio relic is not the only form of radio emission from Abell 4038 that has attracted astronomers' attention. Man Ho Chan and Chak Man Lee at the Educational University of Hong Kong have been intensely scrutinizing the radio emission from Abell 4038 and have indicated that it might be producing excess radio emission that would be consistent with dark matter particles undergoing matter-antimatter annihilation [8, 9].
Anyone who knows a little bit about dark matter will know that no one has identified what dark matter actually is. In most situations, all we can say about dark matter is that it gravitationally interacts with stars and gas but that it normally doesn't emit or absorb any form of electromagnetic radiation. Finding some sort of electromagnetic signal associated with dark matter would be worthy of a Nobel prize.
According to one hypothesis, radio emission could originate when a matter form of dark matter and an antimatter form of dark matter collide with each other. As depicted in cartoons and comic books (or at least some sort of cartoons or comic books that I must have seen at some point that doesn't involve Umamasume), when matter and antimatter collide, they form energy. Any location in space with a lot of dark matter, like a cluster of galaxies, could potentially produce excess radio emission from dark matter and dark antimatter collision, and Chan and Lee at the Educational University of Hong Kong have indicated that Abell 4038 might be producing this form of radio emission.
However, this claim is extremely controversial. To give you an idea of how controversial it is, Chan and Lee's first paper about possibly detecting radio emission from dark matter-antimatter collisions in Abell 4038 was published in 2019 in the Monthly Notices of the Royal Astronomical Society [8] and then retracted by the editors of the journal in 2020 [10]. According to the journal's website, "[T]here is currently insufficient evidence to support the conclusions of the paper. The decision to retract was made by the journal editors following independent criticism of the paper, which was subsequently supported by further peer review by the journal [10]." I had never actually seen a paper retracted from an astronomy journal before. To Chen and Lee's credit, they did get a second paper published later in 2020 in the same journal [9], and that one has not yet been retracted. However, other people still are not quite buying into the claim that dark matter-antimatter annihilation has been detected in Abell 4038. Another group led by Z. Z. Abidin at the University of Malaya have also reviewed the Abell 4038 radio data and have concluded that it is unlikely to be from matter-antimatter annihilation [11].
In any case, I think one thing that I can conclude about dark matter is that I can't conclude anything about dark matter from the Abell 4038 radio data.