Dark Matter: The Great Mystery Of The Universe

Dark matter is one of the most intriguing and mysterious components of the universe. Unlike ordinary matter, which is made up of atoms and is visible through electromagnetic radiation (light), dark matter does not emit, absorb, or reflect light. It makes up 30.1 percent of the matter-energy composition of the universe; the rest is dark energy (69.4 percent) and “ordinary” visible matter (0.5 percent).(Britannica)

Originally known as the “missing mass,” dark matter’s existence was first inferred by Swiss American astronomer Fritz Zwicky who in 1933 discovered that the mass of all the stars in the Coma Cluster of galaxies provided only about 1 percent of the mass needed to keep the galaxies from escaping the cluster’s gravitational pull. (Britannica). He proposed the existence of some unseen mass, which he termed "dunkle Materie" or dark matter, to account for this discrepancy. Decades later, in the 1970s, astronomer Vera Rubin’s work on galactic rotation curves provided further evidence. Rubin found that stars in the outer regions of galaxies were moving at speeds that couldn’t be explained solely by the mass of visible matter, suggesting the presence of additional unseen matter.(Wikipedia)

Dark matter interacts with visible matter primarily through gravity, and it does not participate in the electromagnetic or strong nuclear forces, making it extremely difficult to detect directly.(NASA). Its behaviour can be summarised through the following characteristics:

• Non-luminous: It does not emit, absorb, or scatter light.

• Cold: Most theories suggest dark matter particles are "cold," meaning they move slowly compared to the speed of light, which allows them to clump and form the large-scale structure of the universe.

• Non-baryonic: Dark matter is not composed of baryons, the particles that make up ordinary matter (like protons and neutrons), but is instead thought to consist of unknown particles.

• Clustering behaviour: Dark matter is believed to play a crucial role in the formation of galaxies, with its gravitational pull allowing gas to accumulate and form stars. (Britannica)

  
  

  Several theoretical particles have been proposed to explain dark matter, but none have been definitively detected.

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A simulation of the formation of dark matter structures from the early universe until today. Ralf Kehler/SLAC national accelerator Laboratory, American museum of History

• WIMPs (Weakly Interacting Massive Particles): WIMPs are hypothetical particles that interact only via the weak nuclear force and gravity, making them extremely difficult to detect. WIMPs are one of the leading candidates for dark matter.

• Axions: Another hypothetical particle, the axion, is much lighter than WIMPs and could explain certain astronomical observations.

• Sterile Neutrinos: These neutrinos would only interact via gravity, unlike the three known types of neutrinos that interact via the weak nuclear force. Sterile neutrinos could account for some of the properties attributed to dark matter.

• Extraordinary efforts are under way to detect and measure the properties of these unseen WIMPs, either by witnessing their impact in a laboratory detector or by observing their annihilations after they collide with each other. There is also some expectation that their presence and mass may be inferred from experiments at new particle accelerators such as the Large Hadron Collider. (Britannica). As well as experiments such as LUX-ZEPLIN and XENON1T attempt to detect dark matter particles interacting with ordinary matter in underground detectors shielded from cosmic rays.

Large Hadron Collider CERN near Geneva,Switzerland 

As an alternative to dark matter, modifications to gravity have been proposed to explain the apparent presence of “missing matter.” These modifications suggest that the attractive force exerted by ordinary matter may be enhanced in conditions that occur only on galactic scales. However, most of the proposals are unsatisfactory on theoretical grounds as they provide little or no explanation for the modification of gravity. These theories are also unable to explain the observations of dark matter physically separated from ordinary matter in the Bullet cluster. This separation demonstrates that dark matter is a physical reality and is distinguishable from ordinary matter.(Britannica)

Dark matter continues to be one of the greatest unsolved mysteries in physics. While we have strong indirect evidence for its existence through its gravitational effects, its exact nature remains elusive. Ongoing and future experiments hold the potential to unlock the secrets of dark matter, which would revolutionise our understanding of the universe.

Glossary:

Galactic rotation curve: The rotation curve of a disc galaxy is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. 

Baryons: They are heavy subatomic particles that are made up of three quarks. Both protons and neutrons, as well as other particles, are baryons.

Particle accelerators: They are machines that use electromagnetic fields to propel charged particles to very high speeds and energies to contain them in well defined beams.

Large Hadron Collider (LHC): This is the world’s largest and most powerful particle accelerator. It provides collisions between lead ions, recreating conditions similar to those just after the Big Bang.

Written By: Roshaneh Fatima