A Light Introduction to Dark Matter

The universe in which we live is mind-bendingly expansive - it’s hard enough to get your head around its size without further complicating matters. However, the percentage of the universe that is observable to us – the baryonic matter which we can see through our telescopes – is a trifling portion of that which we believe must be there.

Atoms, which are composed of protons, neutrons and electrons, are the smallest known particle which can make up a chemical element. Amazingly, they only account for around 4.6% of the total mass of the universe. So what comprises the rest? The truth is, it’s impossible to say with any certainty… which is why scientists have taken to naming it “dark matter”.

How Do We Know Dark Matter Exists?

Dark matter does not emit or reflect light, or exude energy. So how do we know it’s there at all? Well, there are several bodies of evidence to support the existence of dark matter.

The first method is very technical and involves studying Big Bang nucleosynthesis. By observing the ratio between helium and hydrogen present in the universe today, scientists can estimate the amount of baryonic matter (that which is composed of atoms) immediately following the Big Bang. And as a consequence, that which should be present today. When equated with the amount of mass we estimate to be present in the entire universe, a huge discrepancy is found, which can only be explained by the presence of dark matter.

Another, simpler means of detecting its presence is far more observational. By studying the movement of neighbouring galaxies, we can see that they are moving much faster than would be expected if their entire mass was composed of that which is visible to us (stars, planets and other luminaries). Therefore, they must contain a significant amount of dark matter. Furthermore, the motion of the stars at the outer reaches of a galaxy cluster should, in theory, be slower than the rest. However, this does not appear to be the case, and scientists have concluded that dark matter is responsible.

What is Dark Matter?

This question is much more difficult to answer, though there are several prevalent theories. Among others, these argue that dark matter is comprised of:

• Normal matter that we have as yet been unable to detect – such as dark galaxies, rock, dust or brown dwarves.
• Massive standard model neutrinos, which could account for the excess mass
• Exotica, such as WIMPs (weakly interacting massive particles), axions, supersymmetric particles, CHAMPs (charged massive particles), and others.

The Future of Dark Matter Research

One of the main problems facing scientists is the difficulty in detecting dark matter. It does not emit or reflect light, making it impossible to see. However, over the past decades, technology has been advancing to try and meet the challenge of detecting dark matter. The Large Underground Xenon experiment, better known as LUX, is the most sensitive dark matter detector to date. In 2012, plans were announced to take the LUX down to the bowels of the Earth in order to seek out dark matter. For more information on this exciting story, see the article: Gold Mine Laboratory Searches for Dark Matter.