Dark matter is a type of matter made up of small particles that do not interact with ordinary matter and do not emit electromagnetic radiation. This means that they cannot generate, reflect or absorb light. For this reason, dark matter is transparent and cannot be seen with the naked eye.
Let us remember that when we speak of matter we refer to anybody or material that occupies a place in space and has mass and volume. Dark matter meets these characteristics even though it is not possible to see it.
So if dark matter is transparent, how do we know it exists? Its existence is known due to the effects it causes in the universe, such as the displacement of stars or the distortion of the light they emit. Both phenomena and others are thought to be influenced by dark matter.
The study of dark matter is of the utmost importance since it would allow us to understand the functioning of stellar structures. It would also help to decipher what happened during the initial moment in which the universe was formed, better known as the Big Bang.
Although the existence of dark matter is a proven fact by science, it is not very clear what it is made of. What is known is that dark matter is not composed of protons or neutrons (baryonic matter). Then, the existence of two fundamental components is proposed:
The non-baryonic matter would be made up of the so-called Weakly Interacting Massive Particles, better known as WIMPs. They are particles smaller than an atom and have the ability to interact with visible matter through the force of gravity.
Their existence is hypothetical and it is believed that if they did exist they would be very cold, would have a large amount of mass, and would move very slowly.
The Massive Galactic Halo Objects or MACHO's (from the English Massive astrophysical compact halo object ) are made up of wandering planets, failed stars, and black holes, which are regions of space with a great gravitational force.
Although it is possible that they are part of the dark matter, their proportion would be very low with respect to the WIMP's.
Dark matter was, at first, just a theory. In 1933, the Swiss astronomer and physicist Fritz Zwicky raised the possibility that there is an undetectable type of matter that could influence the movements that occur in a cluster of galaxies called Coma.
A cluster is a group of stars that attract each other due to their gravitational force. And Zwicky discovered, through the application of the scientific method, that the total mass of the studied cluster was about 400 times greater than what was calculated. For this reason, he deduced that there was a type of non-visible matter that was contributing extra mass to this entire group of stars.
In the 1970s, astronomer Vera Rubin discovered that stars in the Andromeda galaxy moved at the same speed regardless of their location. This uniformity in velocity suggested that there was something else of a non-visible nature that was adding mass to these stars.
Rubin's contribution was fundamental in laying the foundations for the existence of dark matter, which was proven over and over again with various observations. One of the most important was the discovery in 2005 of the galaxy VIRGOHI21, composed almost entirely of dark matter.
In 2021, the most detailed map of the distribution of dark matter was obtained, created by the Víctor M. Blanco telescope, located in Chile. The map shows that dark matter is much more widespread in the universe than initially believed, which opens new approaches in the field of astronomy and physics.
Since dark matter is actually transparent, there is no direct observation method that can easily demonstrate its existence. However, there are other methods. The most used are:
A gravitational lens is a phenomenon that occurs when the light from a very distant stellar body is distorted in the presence of another massive body, such as a galaxy, a planet, the Sun, etc.
The measurement of this distortion, which is in the form of a curve, is used to determine the mass of the object that caused it. The correspondence between mass and luminosity can indicate the presence of dark matter since the higher the luminosity, the more dark matter the studied body contains.
At the center of the Milky Way, our galaxy, there is a very intense halo of light made up of gamma rays called the Galactic Center GeV Excess. And that halo is thought to be generated by dark matter destroying itself.
Although dark matter cannot emit or reflect light, there is a hypothesis that two dark matter particles colliding could generate gamma-ray emission on a very weak scale. This result is called "dark matter annihilation".
Gamma-ray telescopes map the universe in search of the source that causes this type of luminosity and it is suspected that by finding gamma rays, dark matter is also being found. The most famous is the Fermi Gamma-ray Space Telescope.
Although they can be confused as synonyms, dark matter and dark energy are two different concepts.
Dark energy is a form of energy existing in space that exerts such repulsive pressure that it counteracts the effect of gravity and contributes to the accelerated expansion of the universe.
Dark energy makes up almost 70% of the mass in space, but like dark matter, much is still unknown about its function and composition.
Both dark energy and dark matter coexist in the universe, but they are two independent phenomena.