What is dark matter?

Image captured by the 'Hubble' telescope that, according to experts, shows a ghostly ring that could be dark matter.

Dark matter is a matter that does not interact with the electromagnetic field.

That means that we cannot see it, nor is it absorbed by the materials, nor is it reflected.

We know that it exists because there are many clues that astrophysics and cosmology give us.

For example, it is necessary to explain the rotation of objects in galaxies.

Celestial objects rotate much faster around the center of galaxies than if they only followed the classical laws of gravitation because more matter is needed.

Another clue is that to understand the structures of the universe, galaxies, galaxy clusters and other larger structures, we need more matter than normal.

Finally, having mass, it can deform the path traveled by light, it has the effect of gravitational lensing, a phenomenon that we are able to observe with satellites such as

Hubble

.

Unlike dark matter, we and everything we see in the universe are normal matter, made up of known Standard Model particles.

The cosmological explanation that we give to these phenomena that I am telling you is that, in addition to this normal matter, there is a cold dark matter.

Because there is another type of dark matter, well known since the end of the last century, which is neutrinos.

They do not interact with anything, they are like ghost particles, but we know that they are there because we have detected them, although we do not yet know their mass.

However, they are particles that are too light and fast to be at the beginning of the formation of the large structures that we observe in the universe: galaxies, galaxy clusters, etc.

But going back to the dark matter you ask about, we have another clue which is the cosmic microwave background.

As the universe cooled about 300,000 years after the Big Bang, nuclei, very light elements such as hydrogen or helium, formed and electrons revolved around the nuclei, but the density was so great and the distances so small that the photons hitting them took the electrons out of there.

The universe was like a thick fog that couldn't be seen in because photons couldn't get out of it.

That means that no matter how far we look, and the further back is in time, we will not be able to see anything because the photons could not escape.

Until a moment came when everything was magnified and what is called the surface of the last collision happened, the photons were released.

Currently that radiation is a background that is everywhere and that we study.

Analysis of this cosmic microwave background also provides information on the amount of normal and dark matter that exists.

The signs make us a universe in which more than 80% of matter is dark.

What we still do not know is who is responsible for it, what particle or particles make it up

All these signs make us a universe in which more than 80% of matter is dark.

What we still do not know is who is responsible for it, what particle or particles make it up.

And we have been studying it since the sixties of the last century.

In short, we have many clues that this dark matter exists, that it is cold, that is, it is slow, that it became uncoupled at some point in the beginning of the universe's history, but unfortunately we have not found anything yet.

We look for it with different experiments, for example in underground laboratories like the one I work in at Canfranc.

These laboratories are located deep underground because only dark matter is able to get there.

Most of the cosmic rays that reach us stay on the surface of the Earth.

The natural radiation of the site and of the materials themselves remains, which is eliminated using different shielding strategies, clean materials, and analysis techniques, to achieve a signal that has no explanation as normal matter.

But neither we nor any other great international experiment have found anything.

And we have also not found anything conclusive with indirect detection that looks for the traces of possible annihilations or decays of these particles at some point in space.

Here we would get the result: photons or high-energy neutrinos that could detect telescopes such as the MAGIC of the Canary Islands.

And it has not been possible to create the particle that we suppose forms dark matter in experiments of large accelerators such as the LHC at CERN.

We have many clues that this dark matter exists, that it is cold, that is, it is slow, that it became uncoupled at some point in the early history of the universe, but unfortunately we have not found anything yet.

The gravitational interaction causes dark matter to accumulate in a kind of halo that surrounds normal matter (the Sun and us, all normal matter).

But it cannot form objects like a table or anything else because it has no electromagnetic interaction.

The matter that forms us and the rest of the objects in the universe are held together by electromagnetic fields, we are tied to the Earth by gravity, but we have a structure and we do not merge with it due to the electromagnetic cohesion that they have the atoms and molecules (of normal matter) that make up our body or the ground.

That is why dark matter can accumulate but cannot form objects.

Gloria Luzón Marco

is a tenured professor in the Department of Theoretical Physics at the University of Zaragoza, a member of CAPA (Center for Astroparticles and High Energy Physics), and a researcher at the Underground Laboratory of Canfranc (Huesca).

Question sent via email by

Miguel de Armas

We respond

is a weekly scientific clinic, sponsored by the

Dr. Antoni Esteve Foundation

and the

L'Oréal-Unesco 'For Women in Science' program

, which answers readers' questions about science and technology.

They are scientists and technologists, members of

AMIT (Association of Women Researchers and Technologists)

, who answer these questions.

Send your questions to

nosotrasrespondemos@gmail.com

or via Twitter #nosotrasrespondemos.

Coordination and writing:

Victoria Toro

You can follow

MATERIA

on

Facebook

,

Twitter

,

Instagram

or subscribe here to our

newsletter