Dark matter is one of the unsolved mysteries in physics and astrophysics.

This hidden matter, which is believed to form as much as 85% of the total matter in the universe, was first introduced about 60 years ago by the astronomer Fritz Zwicky. He developed and later proposed his “missing matter” theory to explain why stars rotating far away from the center of a galaxy move much more quickly than expected, taking account of the visible mass of the galaxy.

What Zwicky had called missing matter is now commonly called the dark matter. Dark matter cannot be directly observed by any scientific observation devices, but its existence has been suggested by its gravitational effects.

What Is Dark Matter?

As to the nature of the dark matter, there have been many debates and speculations by physicists and some candidates include:

  • Weakly Interacting Massive or Low-Mass Particles. They include particles that have a very weak interaction with the rest of the matter, but with a finite mass contributing to the overall mass of the galaxy or universe. Due to their nature, they cannot be “seen” or easily detected and so can be categorized as dark matter. A good candidate is a tiny ghost particle named neutrino that exists in large numbers throughout the universe. Neutrinos are weakly interacting low-mass particles that because of their abundance in the universe can be a good candidate.
  • Massive Astrophysical Compact Hollow Objects (MACHOS): They include any massive object that can contribute to the dark matter. Black holes, including mini-black holes, that may exist in large numbers in the universe are categorized as MACHO. Contrary to the first candidate, they are very massive and have a high gravitational field. But due to the same reason, they cannot be easily seen as they swallow passing by beams of light.
  • Brown dwarfs are other candidates. They include objects with masses higher than Jupiter but lower than the Sun that virtually do not radiate any light. It is not yet clear as to how many of these bodies exist in the universe and if they contribute to the dark matter.

New Discovery Sheds More Light on the Question of What Is Dark Matter

A new discovery suggests that dark matter may not be as dark as it was previously imagined. Indeed, for the first time and following comprehensive research on four colliding galaxies in a galactic cluster named Abell 3827, researchers claimed to have discovered dark matter to be “slowing down” after interaction with other dark matter.

This phenomenon that contradicts previous investigations suggests that dark matter may be capable of interacting with a force other than gravity. Last August, a team of astronomers led by Dr. Richard Massey of Durham University used the Hubble Space Telescope to take photos of a massive collision between four distant galaxies.

All galaxies are sources of dark matter and more or less the same ratio of %85 applies to every galaxy. The detailed analysis of the collision led the researchers to the conclusion that during the course of the collision, the existing dark matter ended up in a different place to the stars in their associated galaxy.

This suggested that the dark matter might have been influenced by a force other than gravity. Although the dark matter is invisible, it can be detected by the way it bends the light beam emerging from an even more distant galaxy.

More Mysteries around Dark Matter

There are still many other secrets regarding the dark matter that are yet to be resolved. For example, some investigations suggest that colliding dark matter constituents, or so-called dark matter particles, may create an unknown type of radiation named “dark radiation”.

The nature of such radiation if ever exists is yet to be discovered, but the work to find this new radiation has already started and scientists have placed detectors in different underground locations of Earth for detecting dark radiation.

In order to answer the question What is dark matter? and have a clear idea of what its nature and constituents are, much more research has to be done, but the recent discoveries are promising and in the near future we may witness the final answer.


  1. Dark Matter Can Interact With Itself, Galaxy Collisions Show
  2. The Mystery of Dark Matter: WIMPS May Have the Answer
  3. Are Supermassive Black Holes Hiding Matter?
Bob Pershing, M.Sc.

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