A new theory suggests that dark matter is made up of strongly interacting particles that interact through a so-called "dark force." If this is true, this may ultimately explain the extreme density we see in the dark matter halo that orbits galaxies.
There is a type of particle known as self-interacting dark matter (SIDM), which is an alternative to the cold dark matter theory. The cold dark matter theory is a cosmological model that holds that there is an invisible substance in the universe that only interacts with gravity and not with electromagnetic radiation or other matter. The velocity of this substance is low, so it is called cold dark matter. The theory of cold dark matter could explain the large-scale structure of the universe, as well as the formation and rotation of galaxies.
The theory of cold dark matter holds that this elusive matter is made up of massive, slow-moving (and therefore cold), weakly interacting particles that do not collide. The problem with cold dark matter models is that they struggle to explain the two mysteries surrounding the so-called dark matter halo.
The second is that the dark matter halos of superspreading galaxies have extremely low densities, which are difficult to explain with the cold dark matter theory, and superspreading galaxies are bleak, extremely low-density galaxies. Their stars are scattered over a huge area, so the surface brightness of the stars is extremely low, making them difficult to spot. The formation mechanism of super-diffuse galaxies and the nature of dark matter halos are still unclear, and some studies believe that they are caused by the self-interaction of dark matter or the tidal effects of galaxy clusters. Currently, most of the known superspreading galaxies are hidden in larger, brighter clusters of galaxies, but there are also some isolated superspreading galaxies that have been discovered.
Dark matter poses a major puzzle for scientists because, although it occupies about 85% of the matter in the universe, it does not interact with light and is therefore virtually invisible to us. This tells researchers that dark matter can't just be an invisible aggregate of matter made up of electrons, protons, and neutrons — so-called baryonic matter, which includes stars, planets, our bodies, and pretty much everything we see in our daily lives.
In fact, the only way researchers can infer the existence of dark matter is because it has mass and therefore interacts with gravity. This effect can be "felt" by the baryonic matter and light that we are actually able to see.
More specifically, as light passes through these galaxies wrapped in dark matter from the background source, the influence of matter on space-time deflects the path of the light, making the background source appear to "move" to a new location in space.
This effect is known as gravitational lensing, and it is the reason why scientists initially determined that most, if not all, galaxies are surrounded by dark matter halos. Moreover, these halos are thought to be far beyond the limits of the visible material objects of those galaxies, such as stars, gas, and dust.
Gravitational lensing also allows astronomers to measure the density of dark matter halos. A denser halo produces a stronger lensing effect than a less dense halo around a superspreading galaxy. However, researchers have struggled to explain extreme cases of dark matter halo density.
To solve this puzzle, physicists built a high-resolution model of the structure of the universe based on actual astronomical observations to simulate it.
In these simulations, they consider strong dark matter self-interactions at the mass scale associated with strong gravitational lens halos and hyperdiffuse galaxies.
These self-interactions result in heat transfer within the halo, diversifying the halo density in the central region of galaxies, in other words, some halos have a higher central density and others have a lower central density, depending on the cosmic evolutionary history and environment of the individual halos compared to their cold dark matter counterparts.
The team concluded that SIDMs interacting through "dark forces", just as baryon particles interact through electromagnetic forces and strong and weak nuclear forces, can provide a solution that cold dark matter theory cannot provide.
Dark force is a hypothetical force, which is an interaction force between dark matter, and the existence of dark force may explain some strange phenomena of dark matter, such as the extreme difference in density of galactic halos and the self-interaction of dark matter.
The nature of the dark force is still unclear, some studies believe that it is similar to electromagnetic force or strong and weak nuclear force, and some studies believe that it is a completely new force. The detection and verification of dark forces is very difficult, and requires the help of high-precision astronomical observations and particle physics experiments.
Physicists hope that their work will encourage more research in this promising field of research, which would be a very timely development, given the massive influx of data from astronomical observatories in the near future, including the James Webb Space Telescope and the upcoming Rubin Observatory.