# What is Dark Matter and why do scientists believe it occupies most of the Universe?

Physics is an extremely popular topic these days. Neutron stars, black holes, parallel realities, quantum “effects” – these are all topics that are quite exciting and have become very popular in the entertainment world.

But as always, there is often a gap between what physicists do and the information that finally reaches interested people. For this reason, we decided to start some articles in which we will explain in a simple way the nature of some of the great discoveries of modern physics.

Today we will talk about the so-called “Dark Matter”. Surely you have already heard about it, the mystery it represents and how absurd it seems. After all, how is it that most of the universe is made up of a type of matter that we neither know nor understand?

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## Physical models

But before talking about dark matter, we have to explain what a physical model is.

A model is a mathematical system that predicts something. For example: Newton’s famous equation of universal gravitation F1 = F2 = G*[(m1*m2)/r2 is famous not because it is a masterpiece of mathematics (which it was) but because by applying it it is possible to accurately predict the motion of celestial bodies.

Newton’s discovery worked because it served to predict the motion of the planets. And Einstein’s discovery worked because it perfected this and made it functional even for large galaxies and massive bodies. The so-called General Relativity worked well for years, allowing us to adequately predict and calculate the motion of solar systems and galaxies. Until at one point it simply stopped working.

## The galaxy problem

The issue was the following: following the predictions of General Relativity and calculating the mass of galaxies the model stopped working. It turned out that the galaxies should not exist, because the gravity inside them was not strong enough to hold them together, but that their components should have escaped into space long ago.

As always, the first thing was to question the model. Most likely, General Relativity was not an accurate theory and there were better ways to explain the universe around us. Perhaps gravity was not the same, but worked differently in different places in space. It could be postulated, for example, that the more distant a galaxy was from us, the older the reflection of what we see would be and, therefore, that gravity changed as time went by.

But the thing was that none of these hypotheses managed to pass the tests of predicting the behavior of galaxies. And for this reason, scientists had no choice but to postulate a rather simple hypothesis: that galaxies are heavier than we see because they have a part of matter that we cannot see. Hence the term “Dark Matter”.

## The behavior of galaxies

Indeed, observations revealed that different galaxies would have different amounts of Dark Matter. Conversely, if what happened was a change in the way gravity works, we would have all the more distant galaxies behaving in one way and the closer ones in another way.

Worse, a recent discovery revealed the existence of a galaxy whose behavior, at least until recently seen, fits perfectly with General Relativity. This means that this galaxy would have no dark matter and would therefore reveal that it cannot be a modification in the workings of gravity that causes the distortions in other galaxies.

It is difficult for non-physicists to fully understand these debates. But the point is that all the alternatives that have been emerging have proven unworkable, and increasingly the dark matter theory continues to gain traction. It is possible that in the future we will find a different explanation, but for now this seems to be the cause of the distortions in the gravitation of distant galaxies.

## How much dark matter is there?

Dark matter represents approximately 85% of the total matter in the observable universe. That is, of all the matter in the universe, only 15% is visible matter.

## How was dark matter discovered?

Swiss astronomer Fritz Zwicky was the first to propose the existence of dark matter in the 1930s. Zwicky observed that the velocity of galaxies in a galaxy cluster was too high to be explained by the mass of visible matter alone. He concluded that there was a significant amount of invisible matter in the cluster that was gravitationally affecting the velocity of the galaxies.

## How is the search for dark matter going on?

Scientists have been looking for dark matter for decades. One of the ways they do this is by directly detecting dark matter particles. Detectors are placed in subway locations to reduce noise from other particles. Another way is to observe the gravitational effect of dark matter on the cosmic background light, which is the radiation left over from the Big Bang.

## What is the cold and hot dark matter hypothesis?

The cold dark matter hypothesis holds that dark matter particles move slowly and clump together in galactic structures. On the other hand, the hot dark matter hypothesis holds that dark matter particles move at speeds close to the speed of light and are uniformly distributed throughout the universe.

## What are dark matter halos?

Dark matter halos are regions where dark matter is densely concentrated around a galaxy. These halos formed shortly after the Big Bang and are crucial for the formation of galaxies.

## So what is dark matter made of?

The thing is, we have no idea that it could form matter that does not react with the electromagnetic spectrum (this means it does not generate any light or reflect it). Physicists have some theories, but for now they are more in the realm of imagination than certainty. Let’s see:

## Axions

Axions are particles whose existence was first proposed in 1977 and that would essentially be tiny particles that do not interact with photons and that constitute the so-called “cold dark matter”. Although mathematically they could exist, they have never been detected and for this reason we do not know if they are really part of the so-called dark matter of galaxies.

## WIMP

Another alternative proposed is that of the so-called “low-interaction massive particles” (WIMP). In essence, these are massive particles similar to neutrinos in the sense that they do not interact with either electromagnetic energy or the strong nuclear force. However, being larger than neutrinos, they would also be much slower.

As you can see, we still don’t know what dark matter is all about, and we’re not even sure it exists, but it is an important component of large-scale physical models of the universe.

Dark matter could be affecting our galaxy in ways we don’t yet fully understand. For example, astronomers have observed a “stellar stream” of stars that appears to be being torn apart by the gravity of dark matter.

Dark matter is believed to be evenly distributed throughout the universe, forming a “cosmic web” that connects galaxies and galactic clusters. This web is invisible, but astronomers can infer its existence through the gravity it exerts on visible matter.

It has been suggested that dark matter particles may interact with other particles in very subtle ways, which could be the key to their direct detection. Some experiments are looking for these interactions using extremely sensitive materials.

Dark matter may be much older than the observable universe. According to some theories, it could have formed during the Big Bang or even earlier.

Although we cannot see dark matter directly, we can use it for “gravitational lensing”. The gravity of dark matter can distort the light from galaxies behind it, allowing us to map the distribution of dark matter in the universe.

Some theories suggest that dark matter could be composed of “temperate dark matter,” which is a form of dark matter that moves at speeds intermediate between the cold and hot hypotheses. This theory could explain some observations of large-scale galactic structures.

Dark matter is not only important for astrophysics, but also for particle physics. Its existence implies the existence of new particles and forces in the universe, which could be revealing for our understanding of fundamental physics.

## What would be the consequences if dark matter did not exist?

The formation of galaxies and stars would be very different: Dark matter plays a fundamental role in the formation of galaxies and stars. As dark matter clumps into halos, it pulls visible matter toward it through gravity, forming galaxies and stars. If there were no dark matter, this gravitational force would not exist, and the distribution of visible matter in the universe would be very different.

The rotation speed of galaxies would be inconsistent with known physics: Another consequence of the lack of dark matter would be that the rotation speed of galaxies would not conform to known physics. Without dark matter to provide an additional gravitational force, stars at the periphery of galaxies would have to rotate more slowly than we observe. This would suggest that our current theories about gravity and physics in general are wrong.

The expansion of the universe would be different: Dark matter also influences the expansion of the universe. If it did not exist, the expansion rate of the universe would be different than it is today. This would have important implications for cosmology and our understanding of the origin and evolution of the universe.

There would be no gravitational lensing: Gravitational lensing is a phenomenon in which dark matter acts like a lens, bending and amplifying light from distant objects. If there were no dark matter, this phenomenon would not exist, which would mean that we would lose one of our most valuable tools for studying the distant universe.

There would be no dark matter detection experiments: If dark matter did not exist, all experiments that seek to detect it would be useless. This would include direct detection experiments, which seek to capture dark matter particles, and indirect detection experiments, which look for evidence of dark matter in cosmic events, such as the collision of galaxies.

Bibliography ►
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