Dark matter is frustrating because it’s everywhere, but we don’t know what it’s made of. There is plenty of evidence for dark matter, including the rotation curves of galaxies and the cosmic microwave background, but attempts to explain it away as some quirk of gravity fail (Source).
What Is Dark Matter? An Astrophysicist Explains
Evidence for dark matter is found in the rotation curves of galaxies, the temperature of galaxy clusters, the bending of light around massive structures, the large-scale structure of the universe, and the cosmic microwave background.
The matter does not interact with normal matter and is cold and collisionless. It also discovers that every galaxy in the universe is surrounded by a halo of dark matter that shares a common structure called the universal density profile. The video emphasizes that the discovery of dark matter is essential to understanding the dynamics of galaxies and the evolution of the universe.
What does it mean when astronomers say that dark matter is cold?
When astronomers say that dark matter is “cold,” they are referring to the speed at which dark matter particles move. In the context of dark matter, “cold” means that the particles move relatively slowly compared to the speed of light. Specifically, dark matter particles are thought to move at speeds of about 100 to 1000 kilometers per second, which is slow enough to allow the particles to clump together and form large structures such as galaxies and clusters of galaxies.
The term “cold” is used in contrast to “hot” dark matter, which would consist of particles moving at much higher speeds, close to the speed of light. If dark matter were “hot,” it would be much harder for it to clump together and form structures on the scale of galaxies.
It’s worth noting that “cold” dark matter is only one of several theoretical possibilities for what dark matter could be. Other possibilities include “warm” dark matter, which would consist of particles moving at intermediate speeds, and “hot” dark matter, which would consist of particles moving at extremely high speeds. However, the evidence from cosmological observations strongly favors the “cold” dark matter model as the most likely explanation for the observed large-scale structure of the universe.
Dark matter must be cold, collisionless, and abundant. It’s not just normal matter that’s faint and hard to see, like rocks or planets or black holes, because there’s just not enough normal matter to account for all the gravitational effects.
Why is dark matter collisionless?
Dark matter is thought to be collisionless for several reasons (Source 2):
First, it is thought to be composed of weakly interacting massive particles (WIMPs), which have very little interaction with normal matter or with each other beyond the force of gravity.
Second, simulations of the formation and evolution of large-scale structure in the Universe, such as galaxies and galaxy clusters, have been able to reproduce observed features only by assuming the presence of non-collisional dark matter.
Third, observations of the motions of galaxies and galaxy clusters, as well as the large-scale structure of the Universe, suggest that most of the matter in the Universe is non-collisional, with the motions of dark matter indicating that it is smoothly distributed throughout space rather than clumped together by collisions.
What is the best explanation for dark matter?
Dark matter is an elusive and mysterious form of matter that cannot be directly observed, but seems to play an influential role throughout the cosmos. Scientists believe that dark matter makes up most of the mass in the universe, but they are unable to explain its properties.
To gain a better understanding of dark matter, scientists have proposed a number of theories, including weakly interacting massive particles (WIMPs), self-interacting dark matter, primordial black holes, and sterile neutrinos. Evidence for their existence comes from measurements of the gravitational effects on galaxies and from experiments using cryogenic ultracold detectors.
Professor Janna Levin has explored the possibility that heavy neutrinos may make up a substantial fraction of dark matter, which could help explain the “baryon asymmetry” known to exist in our universe. Ultimately, however, it seems that more study is needed if we are to unlock the secrets of this enigmatic form of energy.
Why is dark matter undetectable?
Dark matter is undetectable by current technology and observational methods because it does not interact with light or other forms of electromagnetic radiation, which is the primary way we detect and observe objects in space. Dark matter does not emit, absorb, or reflect light, so it does not produce electromagnetic radiation that can be observed through telescopes (Source 3).
In addition, dark matter does not interact with ordinary matter through the strong nuclear force, the electromagnetic force, or the weak nuclear force, which are the three fundamental forces that govern the behavior of matter at the subatomic level. This means that dark matter particles do not collide or interact with other particles in the same way that ordinary matter does.
The only way we can detect the presence of dark matter is through its gravitational effects. We can observe the gravitational influence of dark matter on the motion of galaxies, clusters of galaxies, and other large-scale structures in the Universe. Scientists use mathematical models to infer the presence of dark matter from the gravitational effects they observe.
While we cannot directly detect dark matter particles, several experiments are currently underway to try to detect them indirectly, through their hypothetical interactions with ordinary matter or through the production of other particles that might be detectable. However, the detection of dark matter particles remains a significant challenge, and the search for dark matter continues to be an active area of research in astrophysics and particle physics.
When was dark matter discovered
The concept of dark matter was first proposed in the 1930s by Swiss astronomer Fritz Zwicky. Zwicky was studying the motion of galaxies in the Coma cluster, a large group of galaxies about 320 million light-years from Earth, and he found that the observed motion of the galaxies could not be explained by the visible matter in the cluster alone. Based on his observations, Zwicky proposed the existence of an invisible form of matter that he called “dark matter,” which he believed was responsible for the observed gravitational effects.
However, it was not until the 1970s and 1980s that the concept of dark matter gained wider acceptance in the scientific community. This was due in part to new observations of the rotational motion of galaxies, which again showed that the observed motion could not be explained by visible matter alone, and to the discovery of dark matter in clusters of galaxies, which showed that dark matter was not just a phenomenon in individual galaxies, but was also present on much larger scales.
Today, the existence of dark matter is widely accepted by astronomers and physicists, and it is thought to make up about 85% of all matter in the universe. However, the nature of dark matter remains a mystery, and scientists are still working to understand its properties and how it interacts with ordinary matter.