As human beings, we tend to think we are at the centre of the universe. However, if scientific discoveries are any indication, this cannot be farther from the truth. In fact, we are just one species on an ordinary planet orbiting an ordinary star out of the billions in an ordinary galaxy out of many in a vast universe. If this wasn’t enough, the matter that constitutes us, the one we can study and see, may only be less than 5% of the constitution of the universe. The other 68% is dark energy and nearly 27% is dark matter (NASA), both of which are still essentially mysteries to us.
In the 1930’s, it was found that the stars of a galaxy only provided a small percentage of the mass that would be required to keep the galaxy together. How did they uncover this information? Gravitational pull as a force is capable of warping space. Light from galaxies bend when they travel by adjacent galaxies. This is referred to as gravitational lensing. The extent of the bending depends on the mass of the galaxy. It was, thus, speculated that there had to be some “missing matter” (now called dark matter) that couldn’t be seen but provided the mass required as per the observation. It was only in the 1970’s that the reality of this missing matter was confirmed.
But what is dark matter? Scientists know more of what dark matter is not, than what it actually is. Firstly, it is known that it’s not just dark clouds of normal matter, because those would emit detectable particles called baryons. Secondly, these aren’t antimatter, since antimatter produces gamma rays when it annihilates with matter, which is not seen in the case of dark matter. Lastly, these aren’t black holes because we do not see enough lensing events to suggest that they are.
What we do know is: 1. There is definitely something there, 2. There is a lot of it, and 3. It can interact with gravity.
The current view of the constituents of dark matter is that it is made of some exotic particles called WIMPs (weakly interacting massive particles) or axions. These do not emit much light, nor do they interact with normal matter, which explains why they have gone undetected for so long.
Dark matter exploration is one of the leading works of research in physics today, with works ranging from detection of dark matter underground (LZ experiment), seeing it in the sky (Fermi Gamma Ray Space Telescope) and even making it in the lab (Atlas experiment).
All the theories that we know about dark matter are just that- theories. We still have a long way to go. However, for now, we can take solace in the fact that we may just be one of the few possible species in the entire universe capable of observing, studying and appreciating the marvels around us and that we live in a time when we have the means to do so.