When you look through a telescope, all you see is light and matter. What if there’s something more than just rocks, gases, and metals; particles more complicated than protons, neutrons, and electrons? What if the visible matter is just a part of what really exists out there and there’s something invisible to the naked eye?
There’s such a particle which constitutes to more than 96% of the matter in the universe and is totally invisible to naked eye. It is also believed that this strange particle hardly interacts with any other particle known to us. Scientists neglected the theory of dark matter until the early 20th century when Hubble made an incredible discovery by proving that the universe has much more to offer than what reaches the eye. It was believed until the 20th century that the universe is no bigger than the Milky Way.
The most naive theory from the 19th century was that everything in the sky belonged to the Milky Way galaxy and it was not until the early 20th century that Edwin Hubble proposed a new theory stating that many visible clusters in the night sky did not belong to the Milky Way, rather they were part of different galaxies like our own.
However, when scientists tried to observe the galaxies, they found that the mass of the galaxies didn’t quite add up. The observable mass was much more than the actual mass. Something was missing in the puzzle that couldn’t be explained by physicists and astronomers; there was something more at play than the optical and radio telescopes could gather. This extra mass was termed as dark matter – something that was unexplainable.
Also, objects rotate faster when they are closer to a larger object. Let’s take the example of our solar system. Mercury has a greater velocity than Pluto as Mercury is closer to the Sun (which is the larger object holding the two objects in their orbits). The effect of gravity – as it is traditionally believed – weakens with distance. Yet, when scientists observed distant galaxies, the orbital velocity of gases and stars on the galactic disc was constant regardless of their distance from the galactic centre.
According to some, there must be 160 times more mass to account for the speeds of the galaxies. This extra mass was in the form of dark matter. Dark matter was violating the laws of physics. It was indirectly holding galaxies together, which would’ve otherwise been ripped apart. There was a halo of invisible matter encircling the galaxies and keeping it whole. Most of the galaxies show similar patterns, they rotate too fast and yet hold together from ripping apart.
Though invisible to modern day telescopes, dark matter can be detected by observing its interactions with light. They can deflect light using their gravity. This phenomenon is also known as gravitational lensing.
Dark matter makes up everything in the universe, you and me and all living things, but it is just not visible.
This leads us to the question: What is dark matter after all and what does it consist of? To answer this, scientists considered other objects that don’t emit light. Black holes or MACHOs (Massive Compact Halo Objects) were considered as they also draw matter by gravitational pull, they also exhibit gravitational lensing. However, there are not enough of them to account for the lost mass. Neutrinos, Axions, and other exotic particles were considered in explaining dark matter, but they were later disregarded as these particles could be recreated using particle colliders and were extremely light. After exhausting all the usual suspects, scientists believe that dark matter is a new exotic particle unlike anything discovered before and billions of them are passing through the earth every second.
The best possible answer to the above question would be to categorize dark matter particles as WIMPS (Weakly Interacting Massive Particles). Although they haven’t been detected, their characteristics match that of dark matter’s very closely.
Scientists have built a laboratory in South Sudan to detect a WIMP by using germanium blocks. Germanium has a very tightly packed nucleus and scientists are hoping to detect a WIMP when it hits the nucleus of germanium atoms. However, as the dark matter passes virtually undetected and hardly interacts with any other particle, scientists have used a technique wherein they have frozen the germanium blocks so that they can detect a dark matter particle by the weak heat signature that it produces when it hits the nucleus of the atoms. Detecting one is no easy job; the odds of a dark matter particle interacting with an ordinary particle is one in a million; and is purely a question of contingency.
If scientists could only discover what a dark matter is, they could uncover many secrets of the universe; what was going on one ten-thousandth of a second after the universe was created, how the universe behaves now, and perhaps even the future of the universe could be predicted.