What is dark matter and why is it still a mystery?

Dark matter is a hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe

Distribution of Dark Matter
This is an image (14 arc min × 8.5 arc min) taken in the direction of the constellation Cancer with Hyper Suprime-Cam (HSC) mounted on the Subaru Telescope. Contour lines indicate the distribution of dark matter, estimated from analysis of the image. (Source: nao.ac.jp)

There are a lot of objects and bodies that exist in this gargantuan universe of ours. Everything in this vast abode that we call the universe, whether big or small, is said to consist of matter. Your phone, your body, your hair, dust, air and everything you see around is matter. Each and every one of these objects consists of matter and their existence can generally be perceived rather easily.

But what if I told you that most of the matter that exists in the universe cannot be perceived? What if I also told you that more than 85% of the matter in the universe has never been observed? These facts are hard to believe and are rather astounding, but, they are, indeed, facts. There is a special kind of matter called dark matter, which constitutes about 85% of all matter and has never been observed directly.


Matter is something that has mass and occupies space. Matter can exist in any form or state. These states are solid, liquid and gas. Matter consists of atoms, or, to be precise, matter is made up of protons, neutrons, and electrons, to be even more precise, matter is made up of quarks and leptons. This matter is generally referred to as ordinary matter.

Fermions: Quarks and Leptons

Alright, so matter is made up of quarks and leptons, but, what are they? Both quarks and leptons are fermions. Fermions are those particles that follow Fermi-Dirac statistics. Fermions do not just consist of quarks and leptons but also consist of the particles that are formed due to the composition of various quarks and leptons. A fermion can, therefore, be elementary, like electrons, quarks, and leptons, or, a composition, like neutrons and protons. Fermions differ from bosons, which, are particles that follow Bose-Einstein statistics.

You may ask, what is the difference between the two? Well, Fermi-Dirac statistics follows Pauli’s exclusion principle and therefore fermions follow Pauli’s exclusion principle, whereas, Bose-Einstein statistics and bosons do not follow Pauli’s exclusion principle. Pauli’s exclusion principle says that identical particles cannot exist in the same quantum state in the same quantum system. Another difference is that bosons have integer spin values whereas fermions have half-integer spin values.

Fermi-Dirac Bose-Einstein Maxwell-Boltzmann statistics. Image Source: Wikipedia

Types of quarks

Quarks are of six types and these six types have their antiparticle counterparts, taking the total to twelve kinds of quarks. These quarks have been named up, down, charm, strange, top, and bottom. Up, charm and top have positive charges of +2/3 each. Down, strange, and bottom have negative charges of -1/3 each. The quarks that have positive charges are called as up-type quarks while the ones with the negative charge are called down-type quarks. Only the top and bottom quarks are stable and constitute normal matter, such as matter consisting of protons and neutrons. Antiparticles of each quark type are present, these are antiup, antidown, anticharm, antistrange, antitop, and antibottom.

Baryons and mesons

Baryons are composite fermions made up of three quarks. Mesons, on the other hand, are composed of two quarks. Protons and neutrons are baryons since they are made of three quarks. Protons consist of two up quarks and one down quark.

Therefore, the charge of a proton is 2/3 + 2/3 -1/3 = +1

Neutrons consist of one up quark and two down quarks.

Therefore, the charge of a neutron is 2/3 – 1/3 – 1/3 = 0

Mesons consist of two quarks, such as the pion. Pion consists of an up quark and a down antiquark.

Hadrons are the family of particles that consist of baryons and mesons.

Types of leptons

Leptons are of six flavors and their antitypes, totaling twelve leptons. These are electron, electron neutrino, muon, muon neutrino, tau, and tau neutrino. The antiparticle counterparts of electrons are called antielectrons or positrons. Of these leptons, only electrons and the neutrinos are stable. Muons and taus are unstable. Electrons and electron neutrino make up normal matter. Neutrinos are neutral and have no charge. These do not interact with other particles in nature and are hard to observe due to their extremely small sizes.

Elementary particles included in the Standard Model
Standard model of elementary particles: the 12 fundamental fermions and 5 fundamental bosons. Brown loops indicate which bosons (red) couple to which fermions (purple and green). (Source: wikipedia.org)

The Standard Model

The standard model hopes to explain the four fundamental forces: electromagnetic forces, gravitational forces, strong and weak forces. The standard model also classifies all elementary particles such as fermions and bosons.

Now, let’s get back to matter. The ordinary matter consists of baryonic matter. Dark matter, on the other hand, is said to consist of non-baryonic particles. Dark matter has never been observed and many scientists are of the belief that dark matter consists of particles that are yet to be discovered. It is hypothesized that without dark matter, galaxies would fly apart instead of rotating. Dark matter also apparently explains how galaxies were formed.

Why galaxies do not fly apart?

Astronomer Vera Rubin, while observing galaxies found that clouds farther away from the center moved faster in their orbits than the ones closer to the center. This contradicts Johannes Kepler’s findings that proved that the farther a planet is from the Sun, the slower it orbits. Measuring the rotations of bodies allows us to calculate their masses and gravity. Therefore, measuring the rotations of clouds in a galaxy would give information of the mass of the galaxy. This meant that there was not enough mass to account for the rapid rotation rates of these clouds.

The only possible explanation for why these clouds moved faster was the existence of dark matter. Dark matter provided the mass and gravity that caused for such rapid speeds of the clouds. Rubin stated that this invisible matter should be at least 5 or 6 times the amount of the visible matter to explain why the clouds moved faster farther away from the center.

Another astronomer, Fritz Zwicky had come up with similar conclusions. He found that the clouds would not stay in the galaxy cluster at the speed in which they were moving. These galaxies should, therefore, fly apart instead of rotating. Zwicky concluded that there must exist dark matter that accounts for this anomalous behavior. Zwicky’s terminology of ‘dark matter’ stuck. He is credited for creating the term.

What could dark matter be made of?

There are several theories that dark matter is made of special subatomic particles called axions. Axions have mass and enough axions could provide the gravity that allows for far off clouds to rotate at high speeds. These axions do not interact with normal matter.

How do we detect dark matter?

Gravitational lensing

A Horseshoe Einstein Ring from Hubble
What’s large and blue and can wrap itself around an entire galaxy? A gravitational lens mirage. Pictured above, the gravity of a luminous red galaxy (LRG) has gravitationally distorted the light from a much more distant blue galaxy. (Source: wikimedia.org)

All matter has mass. Anything that has mass tends to create distortion in the fabric of space-time. Bodies with larger mass, therefore, have better attraction force, or, in other words, they have greater gravitational pulls. Light too bends due to the distortions of these masses. Well, why is it called gravitational lensing? This phenomenon is similar to the bending of light, which, lenses are known for carrying out. Scientists observed the bending of light from galaxies and came up with the conclusion that certain matter out there that forms distortions through its gravity causes some of this bending of light. This matter, as you might have guessed, is dark matter.

Random Quiz

The Pauli's Exclusion principle states that two electrons in same orbitals have:

Correct! Wrong!

The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins.

Why is dark matter still a mystery?

While dark matter is the simplest explanation for the extra gravity and mass that exists, it is not necessarily the correct explanation. There are several theories that claim to explain this extra gravity and mass in the universe. Nobody really knows for sure if the existence of dark matter is a sufficient enough explanation for the existence of the extra mass. Dark matter does not give off light and as I have mentioned, does not interact with particles. Without any interactions, it is extremely hard to derive any conclusions on its nature and properties.

Types of dark matter

The classification of dark matter is based on its velocities. Free streaming length (FSL) is used to describe the distance objects would travel due to the random motions in the early universe. The size of a protogalaxy is used for determining the category of dark matter.

Cold dark matter: Dark matter whose constituents have an FSL less than the size of a protogalaxy.

Warm dark matter: Dark matter whose constituents have an FSL comparable to the size of a protogalaxy.

Hot dark matter: Dark matter whose constituents have an FSL greater than the size of a protogalaxy.

Jonathon Swift, an Anglo-Irish poet once said, “Vision is the art of seeing what is invisible to the others”. Dark matter may be invisible, but it has served to solve a lot of mysteries in this astonishingly mysterious universe. Without the invisible phenomenon of dark matter, there would still be a lot of perplexity regarding the formation of galaxies and their movements. Despite all the information we possess about the universe, nobody can say with certainty that dark matter exists. Perhaps, that is where the magnificence of physics lies, in its mystery, and this mystery is what makes the search for the truth worthwhile.

Read More:

  1. Dark Matter Behaves Differently in Dying Galaxies
  2. Dark matter on the move


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