The search for dark matter (v1.0)

A key reference is Space.com. This is largely an extract. 

Dark matter is the mysterious stuff that fills the universe, but no one has ever seen it. It is believed to be 80% of the matter in the universe. We only assume it exists because, without it, the behavior of stars, planets and galaxies simply wouldn't make sense. Their observed motions do not match their detectable mass. If you include both matter and energy dark matter is 27% of the universe's content with normally detectable matter being 5% of the universe's content and dark energy being 68%. Both dark energy and dark matter are not understood. I wrote an essay on dark energy that covers that subject: https://jaykasi.blogspot.com/2024/01/dark-energy.html

What we do know about dark matter is that if we look at a typical galaxy, take account of all the matter that we see (stars, gas, dust) and use Newton's Laws of Gravity and motion (or, more correctly, Einstein's General Relativity - GR), to try to describe the motions of that material, then we get the wrong answer. The objects in galaxies (nearly all of them) are moving too fast. There should not be enough gravity to keep them from flying out of the galaxy that they are in. The same thing is true about galaxies moving around in clusters.

There are two possible explanations:

1.   There is more stuff (matter) that we don't see with our telescopes. We call this dark matter.

2.   Newton's laws and even GR are wrong on the scale of galaxies and everything bigger. This idea is usually called modified gravity (because we need to modify GR) or Modified Newtonian Dynamics (MOND).

Mostly, cosmologists believe that the answer is that the behavior of galaxies is explained by dark matter. Why? Partly. because it has been very hard to write down a successful theory of MOND or modified gravity. And partly because it turned out that when we turned our microwave telescopes to look at cosmic background radiation (CMB), the light from the early universe, it turned out that, according to GR, the same amount and type of dark matter was also required to explain the behavior of the sound waves that traveled in the universe when it was less than 500,000 years old, and whose imprints we are able to see. Modified gravity struggles to provide a unified explanation across all these systems — galaxies, clusters of galaxies, the universe. But we don't yet know what the dark matter is made of.

Dark matter is completely invisible. It emits no light or energy and thus cannot be detected by conventional sensors and detectors. The key to its elusive nature must lie in its composition, scientists think. Visible matter, also called baryonic matter, consists of baryons — an overarching name for subatomic particles such as protons, neutrons and electrons. Many proposals have been made on what dark matter could be. Examples are non-baryonic particles, and anti-matter. One candidate is a non-baryonic particle called WIMP (weakly interacting massive particles), which are believed to have ten to a hundred times the mass of a proton, but their weak interactions with "normal" matter make them difficult to detect. Another candidate is a particle called neutralinos, massive hypothetical particles heavier and slower than neutrinos, though they have yet to be spotted. There are three known types of neutrinos; a fourth, the sterile neutrino, is also a dark matter candidate. The sterile neutrino would only interact with regular matter through gravity.

Dark matter appears to be spread across the cosmos in a net-like pattern, with galaxy clusters forming at the nodes where fibers intersect. By verifying that gravity acts the same both inside and outside our solar system, researchers provide additional evidence for the existence of dark matter. (Things are even more complicated as in addition to dark matter there also appears to be dark energy, an invisible force responsible for the expansion of the universe that acts against gravity.)

But where does dark matter come from? The obvious answer is that we don't know. But there are a few theories. A study published in December 2021 in The Astrophysical Journal argues that dark matter might be concentrated in black holes, the powerful gates to nothing that due to the extreme force of their gravity devour everything in their vicinity. As such, dark matter would have been created in the Big Bang together with all other constituting elements of the universe as we see it today. Stellar remnants such as white dwarfs and neutron stars are also thought to contain high amounts of dark matter, and so are the so-called brown dwarfs, failed stars that didn't accumulate enough material to kick-start nuclear fusion in their cores.

The search for dark matter is on, on all cylinders!! 

An experiment mounted on the International Space Station called the Alpha Magnetic Spectrometer (AMS) detects antimatter in cosmic rays. Since 2011, it has been hit by more than 100 billion cosmic rays, providing fascinating insights into the composition of particles traversing the universe. "We have measured an excess of positrons [the antimatter counterpart to an electron], and this excess can come from dark matter," Samuel Ting, AMS lead scientist and a Nobel laureate with the Massachusetts Institute of Technology, says. "But at this moment, we still need more data to make sure it is from dark matter and not from some strange astrophysics sources. That will require us to run a few more years."

 Back on Earth, beneath a mountain in Italy, the LNG’s Xenon1T is hunting for signs of interactions after WIMPs collide with xenon atoms. "A new phase in the race to detect dark matter with ultra-low background massive detectors on Earth has just begun with XENON1T," project spokesperson Elena Aprile, a professor at Columbia University, said in a statement. "We are proud to be at the forefront of the race with this amazing detector, the first of its kind."

 The Large Underground Xenon dark-matter experiment (LUX), seated in a gold mine in South Dakota, has also been hunting for signs of WIMP and xenon interactions. But so far, the instrument hasn't revealed the mysterious matter. "Though a positive signal would have been welcome, nature was not so kind!" Cham Ghag, a physicist at University College London and collaborator on LUX, said in a statement. "Nonetheless, a null result is significant as it changes the landscape of the field by constraining models for what dark matter could be beyond anything that existed previously."

 The Ice Cube Neutrino Observatory, an experiment buried under the frozen surface of Antarctica, is hunting for hypothetical sterile neutrinos. Sterile neutrinos only interact with regular matter through gravity, making it a strong candidate for dark matter.

Experiments aiming to detect elusive dark matter particles are also conducted in the powerful particle colliders at the European Organization for Nuclear Research (CERN) in Switzerland.

Several telescopes orbiting Earth are hunting for the effects of dark matter. The European Space Agency's Plank spacecraft, retired in 2013, spent four years in the Lagrange Point 2 (a point in the orbit around the sun, where a spacecraft maintains a stable position with respect to Earth), mapping the distribution of the cosmic microwave background, a relic from the Big Bang, in the universe. Irregularities in the distribution of this microwave background revealed clues about the distribution of dark matter.

 In 2014, NASA's Fermi Gamma-ray telescope made maps of the heart of our galaxy, the Milky Way in gamma-ray light, revealing an excess of gamma-ray emissions extending from its core. "The signal we found cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models," lead author Dan Hooper, an astrophysicist at Fermilab in Illinois, says. The excess can be explained by annihilations of dark matter particles with a mass between 31 and 40 billion electron volts, researchers said. The result by itself isn't enough to be considered a smoking gun for dark matter. Additional data from other observing projects or direct-detection experiments would be required to validate the interpretation.

 The James Webb Space Telescope launched after 30 years of development on Dec. 25, 2021, is also expected to contribute to the hunt for the elusive substance. With its infrared eyes able to see to the beginning of time, the telescope of the century won't be able to see dark matter directly, but through observing the evolution of galaxies since the earliest stages of the universe, it is expected to provide insights that have not been possible before.

  ESA's Euclid mission launched on July 1, 2023, and is currently on the hunt for dark matter and dark energy. The mission aims to map the geometry of matter in the universe, specifically the distribution of galaxies to learn more about the elusive dark matter.