Dark matter is a form of matter that does not interact with light but exerts gravitational effects. Its existence is hypothesized to explain certain astronomical observations.
Scientists must consider the additional matter, as the gravitational effects in the cosmos cannot be fully explained by Newton's laws or general relativity. There is more gravitational force at work than what we can detect with normal (baryonic) matter. Therefore, we need to account for more matter to explain these effects. These phenomena occur in a particular context.
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By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=39500241 |
Formation and evolution of galaxies:
Without the influence of dark matter's gravitational force, the galaxies would not form or hold together as they are.
gravitational lensing:
As the gravity of ordinary matter bends light, dark matter bends light as well, causing a phenomenon called gravitational lensing. Due to the gravity of dark matter, light from distant objects can be warped, altering their shapes and structures.
the observable universe's current structure:
This large-scale structure of the universe, as we see with its clusters and voids, can only be shaped by more matter than is seen. The clumping and gaps in the universe's overall structure wouldn't be possible without the gravitational influence of dark matter.
mass position in galactic positions:
During collisions between galaxies, the observed motion and observed outcomes cannot be explained by just visible matter; therefore, dark matter's gravitational influence plays a vital role in these cosmic collisions. During these smash-ups, the distribution of dark matter determines how matter interacts and emerges.
the motion of galaxies within galaxy clusters:
The orbits of galaxies within clusters are shaped by the combined gravity of all the matter. Galaxies don't just move randomly.
microwave background anisotropies.
Tiny temperature variations in the cosmic microwave background radiation, a fossil remnant of the Big Bang, are influenced by dark matter.
Even though dark matter does not emit light, it affects the CMB radiation through gravitational influence. These phenomena leave telltale clues about their distribution and properties.
How much dark matter is there?
According to the standard Lambda-CDM model of cosmology and according to scientific observations, the universe is made up of three different components: ordinary matter, dark matter, and dark energy. Ordinary matter makes up only 5% of the total mass-energy content of the universe, while dark matter constitutes 26.8% and dark energy constitutes 68.2%. This means that the total mass constituted by dark matter is 85%, while dark energy and dark matter together constitute 95% of the total mass-energy content. Ordinary matter, on the other hand, constitutes only a small fraction of the mass of the universe.
matter, Ordinary matter is known as baryonic matter, and this dark matter does not appear to interact with it or receive radiation except through gravity. It is hard to detect such matter in the laboratory; among the explanations, the most prevalent is that it is some kind of as-yet undiscovered sub-particle like axions and WIMPs (weakly interacting massive particles).
Another explanation could be that they are composed of primordial black holes.
Read this article so that you do not confuse dark matter with dark energy. Dark Energy | Explanations for Dark Energy | How much dark energy is in space?
Classifications of dark matter (according to velocity):
According to the velocity (free streaming length) of dark matter, it is classified as "cold," "warm," and "hot.".
Cold, dark matter:
In the early universe, these particles were slow-moving and very rarely interacted.
Compared to the speed of light, CDM has low velocities, which allow the formation of large-scale structures like galaxies and galaxy clusters. WIMPs and axions fall under this category.
Warm dark matter:
WDM has intermediate velocities, impacting structure formation on smaller scales.
They have less velocity and interaction compared to HDM in the early universe. Sterile neutrinos are being considered as potential candidates for warm dark matter.
Hot dark matter:
HDM has high velocities. These particles were relativistic (traveling near the speed of light), and their interaction with other particles was frequent in the early universe.
Due to its high velocity, it tends to smooth out structures, which makes it less likely to form galaxies on small scales.
An example can be neutrinos, but their abundance seems insufficient to explain dark matter.
Beyond cold, dark matter reigns:
Current models have favored the idea of a cold dark matter scenario in which structures form through the gradual accumulation of particles, but after a half-century of unsuccessful searches for dark matter particles and more recent gravitational waves, James Webb space telescope observations have significantly bolstered the argument for the existence of primordial and direct collapse black holes.
Glossary
WIMPs:
Weakly interacting massive particles are hypothetical particles. They are considered to be one of the leading candidates for dark matter.
- They are thought to be more massive than protons, like 10 to 10000 times as massive.
- They rarely interact with ordinary matter, primarily through the weak nuclear force and gravity.
- They are non-luminous, meaning they do not absorb or reflect light.
WIMPs are elementary particles. Elementary particles are subatomic particles that are considered to be the building blocks of matter, as they are not made up of other particles.
Axions:
They are another hypothetical elementary particle, like WIMPs.
Axions were first introduced in 1977 by Peccei and Quinn to solve the theoretical problem called the Strong CP problem in particle physics.
They are thought to be
- Extremely lightweight, potentially millions to billions of times lighter than electrons.
- They interact faintly with photons and matter.
- Axons are bosonic, similar to photons; they can clump together and form a kind of quantum fluid.
The existence of axions in a specific mass range would make them prime candidates for the composition of cold dark matter.
Primordial black holes:
PBHs are a hypothetical type of black hole that could have been formed just after the Big Bang. They aren't created by the collisions of massive stars like ordinary black holes.
These black holes could have formed from extremely dusty particles of matter much earlier and can be as tiny as atoms. Imagine a grand piece of sand having the mass of a mountain. These objects are potential candidates for dark matter.
Neutrinos:
Neutrinos are fascinating little particles. They are incredibly tiny, smaller than neutrons and electrons.
They are neutral, meaning they have no electric charge, neither positive nor negative, making them difficult to manipulate directly.
They are very light and interact very little with other matters.
Sterile neutrinos:
Sterile neutrinos are a hypothetical form of particle that belongs to the neutrinos family, but they do not interact with weak nuclear forces. If they exist, they will only interact through gravity, unlike electrons, muons, and tau neutrinos. The name "sterile" indicates their lack of participation in weak interactions.
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