Most of the Universe consists of “matter” that cannot be seen, possibly immaterial, and interacts with other things only through the force of gravity. Oh yes, and physicists do not know what this matter is or why there is so much of it in the Universe - about four-fifths of its mass.
Scientists call it dark matter.
So where is this mysterious matter that makes up such a huge chunk of our universe, and when will scientists discover it?
How do we know this matter exists
The hypothesis of dark matter was first put forward by the Swiss astronomer Fritz Zwicky in the 1930s, when he realized that his measurements of the masses of galaxy clusters showed some of the mass in the Universe “missing”. Whatever makes galaxies heavier, it does not emit any light, nor does it interact with anything other than through gravity.
Astronomer Vera Rubin, in the 1970s, discovered that the rotation of galaxies does not follow Newton's Law of Motion; stars in galaxies (in particular Andromeda) seemed to rotate around the center at the same speed, but those farther from the star move more slowly. As if something adds mass to the outer part of the galaxy that no one could see.
The rest of the evidence comes from gravitational lensing, which occurs when the gravity of a large object bends light waves around an object. According to Albert Einstein's theory of general relativity, gravity bends space (like a sumo wrestler can deform a mat on which he is standing) so that light rays bend around large objects, even though light itself is massless. Observations showed that there was not enough visible mass to bend the light as it did around individual galaxy clusters - in other words, the galaxies were more massive than they should be.
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Then there is the relic radiation (CMB), the “echo” of the Big Bang and supernovae. “The CMB tells us that the universe is spatially flat,” said Jason Kumar, professor of physics at the University of Hawaii. “Spatially flat” means that if you draw two lines through the universe, they never intersect, even if the lines were billions of light-years across. In a steeply curved universe, these lines will meet at some point in space.
There is now a small controversy among cosmologists and astronomers as to whether dark matter exists. It does not affect light, and it is not charged like electrons or protons. Until now, it has eluded direct detection.
“This is a mystery,” said Kumar. There may be ways that scientists have tried to "see" dark matter - either through its interaction with ordinary matter, or through looking for particles that could be dark matter.
What dark matter is not
Many theories have come and gone as to what dark matter is. One of the first was quite logical: the question was hidden in massive astrophysical compact halo objects (MACHOs), such as neutron stars, black holes, brown dwarfs and rogue planets. They do not emit light (or they emit very little), so they are virtually invisible to telescopes.
However, exploring galaxies looking for small distortions in starlight produced by MACHO passing by - called microlensing - could not explain the amount of dark matter around galaxies, or even much of it. “MACHOs seem to be as excluded as ever,” said Dan Hooper, an associate researcher at the Fermi National Accelerator Laboratory in Illinois.
Dark matter does not appear to be a cloud of gas that cannot be seen through telescopes. Diffuse gas will absorb light from galaxies that are farther away, and at the top of that normal gas will re-emit radiation at long wavelengths - there will be a huge emission of infrared light in the sky. Since this does not happen, we can rule it out.
What could it be
Weakly interacting massive particles (WIMPs) are some of the strongest contenders for the explanation of dark matter. Wimps are heavy particles - about 10 to 100 times heavier than the proton, which were created during the Big Bang and remain in small numbers today. These particles interact with normal matter through gravity and weak nuclear forces. The more massive WIMPs will move more slowly through space, and therefore may be candidates for “cold” dark matter, while the lighter ones will move faster and be candidates for “warm” dark matter.
One way to find them is through “direct detection,” such as the Large Underground Xenon (LUX) experiment, which is a container of liquid xenon in a South Dakota mine.
Another way to see wimps could be with a particle accelerator. Inside accelerators, atomic nuclei are broken at a speed close to the speed of light, and in the process this collision energy is converted into other particles, some of them are new to science. So far nothing has been found in particle accelerators that looks like putative dark matter.
Another possibility: axions. These subatomic particles could be detected indirectly by the types of radiation they emit, how they destroy or how they decay into other types of particles or appear in particle accelerators. However, there is no direct evidence for axions either.
Since the discovery of heavy, slow “cold” particles like wimps or axions has yet to produce results, some scientists are looking at the possibility of light, faster moving particles that they cause “warm” dark matter. There has been renewed interest in such a model of dark matter after scientists found evidence of an unknown particle using the Chandra X-ray Observatory, in the Perseus cluster, a group of galaxies about 250 million light-years from Earth. The known ions in this cluster produce certain lines of X-ray emission, and in 2014, scientists saw a new “line” that could correspond to an unknown light particle.
If dark matter particles are light, scientists will have a hard time finding them directly, said Tracey Slater, a physicist at MIT. She proposed new types of particles that can make up dark matter.
“Dark matter with a mass below about 1 GeV is really difficult to detect with standard direct detection experiments because they work by looking for unexplained recoils of atomic nuclei … but when dark matter is much lighter than an atomic nucleus, the recoil energy is very small,” Tracy said Slater.
Much research has been done in the search for dark matter, and if current methods fail, new ones will be conducted. Using “liquid” liquid helium, semiconductors and even breaking chemical bonds in crystals are some of the new ideas for detecting dark matter.