How much scientists know about dark matter?

What is dark matter?

Dark matter was created during cosmic inflation.
Although scientists agree that dark matter plays a crucial role in the formation of galaxies and galaxy clusters , no proof of the origin of the mysterious matter has so far been produced.

Not long after the physicists of the Large Hadron Collider (LHC) at CERN discovered the Higgs boson, CERN's CEO Rolf-Dieter Heuer wondered what the next step would be, and as one of the top priorities he cited: discovering dark matter.

Dark matter is five times more prevalent than ordinary matter. It seems to exist in clusters all over the universe, becoming perceptible in galaxies. The nature of dark matter is unknown, but physicists suggest that it, like visible matter, is made of particles.

Dark matter appears periodically in the media, often when an experiment detects a possible sign of it. But we're still waiting for the Nobel Prize to come when scientists finally find it. So, how do we know dark matter exists? There are some known facts about dark matter.

What is the evidence dark matter exists?

Scientist have already discovered dark matter and possibly observed it. However, scientists don't know what is dark matter made of, there might be several types of dark matter composition. Finally, the odds are scientists will observe dark matter in the next 5 or 10 years, and understand it better, but not in its entirety.

1. Scientists have already discovered Dark Matter

Currently several experiments are in search of dark matter. But scientists have already discovered its existence decades ago.

In the 1930s, astrophysicist Fritz Zwicky observed the rotations of the galaxies that make up the Coma cluster, a group of over 1,000 galaxies located 300 million light years from Earth. He estimated the mass of these galaxies based on the light they emit. 

He was surprised to find that, if his estimate was correct, at the speed the galaxies were at, they would have to separate. In fact, the cluster needed 400 times more mass than had been calculated to hold together. Something mysterious seemed to be at work in the process: unobserved "dark" matter seemed to be adding mass to the galaxies.

The idea of dark matter was largely ignored until the 1970s, when astronomer Vera Rubin observed something that gave her the same thought. She was studying the speed of the stars moving around the center of our neighboring Andromeda galaxy. She anticipated that the stars at the edges of the galaxy should move more slowly than those at their center because the stars closest to the bright (and therefore massive) cluster of stars at the center should feel a greater gravitational pull.

However, she found that the stars on the edges of the galaxy moved as fast as those in between. This would make sense, she thought, if the disc of visible stars were surrounded by an even larger circle made of something she could not see: something like dark matter.

Other astronomical observations have since confirmed that something strange is happening in the way galaxies and light move through space. It is possible that our confusion is due to a flaw in our understanding of gravity, Rubin herself says she prefers this idea. However, if it is true that dark matter exists, we are already able to see its effects.

2. Scientists have possibly already observed Dark Matter

Many experiments are searching for dark matter, and some of them may have already found it. The problem is that no experiment has been able to claim the discovery with enough confidence to convince the scientific community, either because of statistics or the inability to rule out possible alternative explanations. And none of the claims have convincingly aligned themselves for scientists to declare any results as confirmed.

In 1998 scientists in the DAMA/NaI experiment, a dark matter detector in the depths of the Gran Sasso mountain in Italy, observed a promising pattern in their data. The rate at which the experiment detected signals of possible dark matter particles changed throughout the year, rising to its peak in June and decreasing to lower points in December.

That's exactly what the DAMA scientists hoped to find. If our galaxy is surrounded by a dark matter envelope, the Earth is constantly moving through it as it orbits the Sun, and the Sun is constantly moving through dark matter as it orbits the center of the Milky Way. For half the year, the Earth is moving in the same direction.

During the other half, it is moving in the opposite direction. When the Earth and Sun are moving together, their combined speeds through the dark matter envelope are faster than the Earth's speed when it and the Sun are at odds. The results of DAMA seem to have revealed that the Earth is actually moving through a dark-matter envelope.

However, there are some gaps; the particles observed by DAMA dark matter detector may be something other than dark matter, something through which the Earth and Sun are constantly moving. Or something could be changing the environment nearby. The DAMA experiment, now called DAMA/LIBRA, continues to observe this annual modulation, but the results are not conclusive enough for most scientists to consider this as a discovery of dark matter.

It will be difficult for any experiment to convince scientists that they have found dark matter. That may happen when several experiments start to see the same thing. Dark matter may turn out to be something weirder and more complicated than scientists expect.

In 2008 the PAMELA, a cosmic ray detector in space, detected an excess of positrons, which could be the result of dark matter particles colliding and annihilating each other. In 2013 the Alpha Magnetic Spectrometer (AMS) experiment, attached to the International Space Station, found the same result with an even greater level of certainty. But scientists remain unconvinced, arguing that positrons may be coming from pulsars.

Underground dark matter experiments (CoGeNT, XENON, CRESST, CDMS and LUX) have sometimes supported possible observations of dark matter. It seems that we will need to wait until the next generation of dark matter experiments is complete to get a clearer picture.

3. Scientists do not know what is dark matter made of; there may be several types that make up dark matter.

Scientists have created several models of how dark matter should appear. The main candidate currently is the so-called WIMP, weakly interacting massive particle. Other possible ones include particles already conveniently predicted in supersymmetric models, a theory that adds a new fundamental particle corresponding to each one already known. Groups of scientists are also looking for dark matter particle called axion.

But there is no reason to believe that there is a single type of dark matter particle. The visible matter, the quarks, gluons and electrons that compose everything around us, together with an entire zoo of particles and fundamental forces, including the photons, neutrinos and Higgs bosons, compose only 5% of the universe. The rest is dark matter, which makes up 23%, and dark energy, another mystery in physics that claims the remaining 72%.

4. There's a good chance scientists will observe dark matter for the next 5 or 10 years, but we they never see it completely.

With a number of different experimental ideas programmed to become reality in the coming years, many predict that dark matter will be within our reach within a decade. All of the new techniques to detect dark matter are coming of age at the same time.

🔹 Scientists can detect dark matter in many different ways.

First, they can detect dark matter directly. Direct detection involves waiting patiently with a large, sensitive experiment in a quiet, underground laboratory as free as possible from potential interference from other particles. In the coming years, scientists will shorten their current list of detection technologies to focus their resources on building the largest and most sensitive experiment to date.

The second way to find dark matter is to observe it indirectly, searching for its effects through experiments located in space. Updates to current satellite or International Space Station experiments will give scientists more data to help them determine the significance of possible dark matter effects they have observed.

The third way to find dark matter is to produce it on an accelerator like the Large Hadron Collider (LHC) at CERN. It is possible that when two beams of particles collide in the LHC, their energies convert into mass in the form of dark matter.

But it's also possible that the dark matter is out of our reach, unable to be detected or produced. If scientists do not observe dark matter for the next 10 years, they may need to find a new way to look for it. Or maybe we need to reconsider what we know about gravity.