What is dark matter?
About 80 percent of the world’s mass is made up of material that scientists cannot directly observe. This strange substance, known as dark matter, does not emit light or energy. So why do scientists think most of the world is made of this material?
For the first time, studies of other galaxies in the 1950s showed that the universe has more matter than what is seen with the naked eye. Over the years, scientific support for the theory of dark matter has increased, and although there is no direct evidence for the existence of dark matter, there are strong possibilities.
“The motion of the stars determines the amount of matter in the universe,” said Pieter van Dokkum, a researcher at Yale University. “Stars do not matter what matter exists in the universe, they just tell you that it exists.” van Dokkum is the person who led the team that discovered the Dragonfly 44 galaxy. This galaxy is composed almost entirely of dark matter.
The known constituent of the universe, called baryon, is made up of protons, neutrons and electrons. Dark matter can be made of baryon or non-baryon matter. In order for dark matter to hold the elements of the universe together, it must make up about 80 percent of the matter in the universe.
The missing substance may consist of the same ordinary baryon substance that is only slightly more difficult to detect. Possible options include light brown dwarfs, white dwarfs and neutron stars. Supermassive black holes can also be part of this mass difference. But these super-black holes, which are very difficult to observe, must play a much more colorful role than what scientists have observed in order to form the lost mass of the universe. These are while other factors indicate that dark matter has an external origin.
Most scientists think that dark matter is made of non-baryonic matter. The main option in this field are heavy particles with weak interaction (WIMPs). The mass of these particles is 10 to 100 times that of protons, but their weak interaction with ordinary matter makes them difficult to detect. Neutralinos are hypothetical heavy particles that are heavier than neutrinos but slower than them. Although these particles have not yet been observed, neutralinoes are nevertheless the main candidate for dark matter.
Sterile neutrinos are another option. Neutrinos are particles that do not exist in the structure of ordinary matter. A stream of neutrinos is emitted from the sun, but because these particles rarely interact with ordinary matter, they easily pass through the earth and its inhabitants. There are three known types of neutrinos; The fourth type, called sterile neutrinos, is considered a possible dark matter option. Sterile neutrinos will interact with ordinary matter by gravity.
“One of the fundamental questions is whether there is a pattern for the percentage of each neutrino or not,” Tyce DeYoung, an associate professor of physics and astronomy at Michigan State University and a participant in the IceCube experiment, told Space.com. .
Smaller neutral particles of axons as well as photons are other candidates for dark matter.
According to a statement from the National Gran Sasso Laboratory (LNGS) in Italy, “Several spatial measurements have confirmed the existence of dark matter, leading to a global effort to directly observe the interaction of dark matter particles with ordinary matter in highly sensitive detectors.” Is. This confirms the existence of dark matter and reveals its properties. “However, these interactions are so weak that they have so far avoided direct detection, forcing scientists to make more sensitive detectors.”
Of course, there is a third possibility, and that is that the laws of gravity, which have so far successfully described the motion of objects within the solar system, need to be revised.
Proof of what has not been seen
If scientists cannot see dark matter, how do they know it exists?
Scientists calculate the mass of large objects in space by examining their motion. Astronomers studying spiral galaxies in the 1950s expected matter to move faster in the center of the galaxy than at its sides. But they observed that the stars were moving at the same speed in both places, indicating that the galaxies had more mass than could be seen. Studies on gas inside elliptical galaxies also show the need for more mass than is seen in space objects inside that galaxy. If the galaxy clusters had only a mass measured by conventional astronomical measurements, they would be easily separated from each other.
Albert Einstein showed that massive objects in the universe bend light, so they can be used as lenses. By studying how light is transformed by galaxy clusters, astronomers have been able to map the dark matter in the universe.
All of the above methods provide strong indications that most of the constituents of the universe are substances that have not yet been observed.
Although dark matter is different from ordinary matter, there are currently several research experiments working on how to detect this unusual substance.
The Alpha Magnetic Spectrometer (AMS), a highly sensitive particle detector on the International Space Station, has been operating continuously since its installation in 2011. So far, AMS has been able to record more than 100 billion cosmic ray collisions with its detectors. “Nobel Prize-winning AMS scientist Samuel Ting from MIT says:
We have recorded an excessive amount of positrons (the antimatter counterpart of an electron), and this excess amount of positrons could be due to dark matter. But for now, we still need more data to make sure it comes from dark matter and not from another strange, unknown astronomical source. “That’s why we need to continue our experiment for a few more years.”
Going back to Earth, under a mountain in Italy, the XENON1T research project at the LNGS lab seeks to capture signs of interaction after WIMPs collide with xenon atoms. The lab recently published the first results of this experiment.
“We are very proud to be at the forefront of this scientific competition to prove the existence of dark matter,” said Columbia University professor Elena Aprile, a professor at Columbia University, in a statement. .
The Large Dark Matter Xenon-Dark (LUX) underground experiment at a gold mine in South Dakota is similarly looking for signs of an interaction between WIMPs and xenon. To date, this tool has not been able to reveal this mysterious substance.
“Although a positive signal made us very happy, nature was not so kind to us,” said Cham Ghag, a physicist at University College London and a participant in the LUX research project. However, a neutral result is also very important and changes the overall scheme of the discipline because these results limit the models used to describe dark matter beyond what has ever existed.
The IceCube Neutrino Observatory, an experiment buried under Antarctic ice, seeks to capture sterile neutrinos. Sterile neutrinos interact with ordinary matter only through gravity, making them a strong choice for dark matter.
Other tools do not look for dark matter but for its effects. The European Space Agency’s Planck spacecraft has been mapping the universe since its launch in 2009. By observing how the universe’s mass interacts, the spacecraft could study dark matter and its other partner, dark energy.
In 2014, NASA’s Fermi Gamma-ray Space Telescope mapped the heart of the Milky Way with gamma rays. These maps showed the release of large amounts of gamma rays from the core of the Milky Way.
“The signal we found cannot be justified by other arguments, but it is very close to the predictions of the simplest dark matter models,” said Dan Hooper, an astrophysicist at Fermilab, Illinois.
The researchers say that these large amounts of gamma rays can be explained by the hypothesis that dark matter particles with masses between 31 and 40 billion electron volts are destroyed. These results are not clear evidence for the existence of dark matter because additional data from other projects or direct detection experiments are needed to validate the above interpretation.
Dark matter versus dark energy
Although dark matter makes up most of the universe, it is only about a quarter of the composition of the universe. Much of the universe is made up of dark energy.
After the Big Bang, the world began to expand outward. Scientists once thought that the energy of the universe would eventually run out and slow down over time as gravity pulls the objects inside it towards each other. But research on distant supernovae has shown that the universe is expanding faster (not slower) than ever before, indicating that the process of expansion is accelerating. Is catching. This phenomenon is possible only when the universe has enough energy to overcome gravity, which is called dark energy.