Shedding Light On Dark Matter

Think back to your days as a child, when an adult, for the very first time, tells you that a seemingly empty room is in fact, not empty but rather filled with invisible gases that we can`t see, hear, touch, smell, or feel. Maybe your reaction was awe and wonder. Maybe it was a faint disinterest. Whatever it was, the important fact is that you learnt for the first time that materials which could not be physically perceived existed in this world.

Learning about dark matter for the first time is very much a similar experience, except on astronomical scales. Apart from the many stars and planets that can physically be observed in the universe, the vast expanse of deep space is not simply an empty vacuum as previously thought.

Much like the existence of air can be deduced by laboratory experiments, the existence of dark matter, a substance that we cannot see, feel, or detect, has been hypothesised.

The "missing mass problem" of 1933 was what gave rise to this hypothesis. Fritz Zwicky, a Swiss astronomer, was trying to measure the mass of the Coma galactic cluster (a group of galaxies) when he noticed something extraordinary. The gravitational mass of the galaxies within the cluster that he had calculated was 400 times too large compared to the total light emitted from it. He concluded that most of the mass in the cluster must be "dark"- or not emitting or reflecting any light. He referred to the inference of this unseen matter as dunkle Materie- now known as dark matter.

However, his ideas were not taken very seriously at the time. It was only when an American astronomer, Vera Rubin at the Carnegie Institute for Science, provided concrete evidence for dark matter in the 1970s that the world began to take notice. She had been comparing the orbital speeds of stars in the Andromeda galaxy with their distance from the galaxy`s centre point, when she also came to a dilemma much like Zwicky. Because the edges of the galaxy were sparsely populated with stars, she had predicted that those stars did not feel as much gravitational force as the ones closer to the centre. Therefore, in principle, they had to be orbiting slower than the stars nearer the core of the galaxy.

However, her recordings did not stay true to her expectations- no matter where the star was situated in the galaxy, its orbital speed remained consistent.

She named this the "galaxy rotation problem". Her calculations revealed that ten times more mass was needed for there to be enough gravitational force for the outer stars to travel at equal speeds to the inner stars- and yet, the Andromeda galaxy was not falling apart. After 60 other investigations gave the same result, Rubin was convinced- a colossal amount of unseen matter must populate galaxies, invisible to the eye but exerting gravity all the same. "What you see in a spiral galaxy" Rubin concluded, "is not what you get."

Initially, the scientific community was reluctant to recognise this outcome. However, her observations were so detailed, and the conclusion so obvious, that people realised that it had to be right. The search for dark matter had begun.

Several different large scale experiments have been dedicated thus far towards isolating the invisible particle, such as the Large Underground Xenon Experiment (LUX) based in South Dakota, USA, and the Large Hadron Collider based in Europe, all using different methods to solve the mystery. LUX uses 370kg of liquefied xenon as a detector in order to test for nuclear recoils that may occur when a dark matter particle and the nucleus of a xenon atom collide. The LHC, a more direct detector, smashes protons at massive velocities, recreating the early moments of the universe`s creation, and collecting data on the substance given out.

Yet, in the few decades that the experiments have been running, nothing has been revealed.

Embarrassingly, dark matter so far has lived up to its name; its true nature has remained elusive to modern day science. Despite the fact that it has been estimated to make up 85% of total matter in the universe (a rather sobering figure), finding the candidate that fits the category has been difficult to say the least.

However, it`s too early to lose hope. Even the limited progress that we have made so far has enabled us to break down the universe to its fundamental particles; what we may achieve in the future is unfathomable. Although the expanse of the universe and its secrets may belittle human understanding, every day brings us another clue to solve its mystery. All we have to do is wait, and receive.

"The universe is full of magical things patiently waiting for our wits to grow sharper." -EDEN PHILLPOTTS-