The universe is around 13.8 billion years old, and we haven’t even started to understand its makeup despite scientific and technological advancements. When we think of the makeup of our universe, we automatically think of the stars, planets and galaxies, which rotate gracefully in space. But in the 1960s and 1970s there were observations and discoveries that changed our understanding of the cosmos. The matter that we see and experience in our daily life, which is made up of atoms and therefore considered ordinary, is actually quite rare. Matter as we know it comprises less than five percent of the universe. The rest of the universe, 95 percent, is the dark universe, made up of dark matter (about 25 percent) and dark energy (about 70 percent). This discovery is largely attributed to astronomer Vera Ruben. When light is incident on an object, part of it is reflected and when the reflected light enters our eye, we are able to see the object. However, when light falls on dark matter, it simply passes through it. Light or any other radiation or electromagnetic matter does not interact with dark matter and therefore cannot be seen. (Probably this should have been called invisible matter). At this very moment, particles of dark matter are passing through us. What we know about dark matter is that it exists, and that’s it. The hunt for dark matter has gone on for decades. Although we cannot see, smell, taste, or hear it, we can observe its gravity impacting the visible universe.
There is a lot of evidence that supports the existence of dark matter. We know that the planets closest to the sun rotate much faster than the outer planets. When we apply the same logic to galaxies, parts near its center should spin faster than parts far away. But the measurements have conclusively proven that the outer parts move as fast as the inner parts of galaxies. The logical explanation put forward for this observation is that galaxies have enormous amounts of dark matter, which provides the additional gravitational pull to ensure that visible matter throughout the galaxy spins at the same rate. Is dark matter an exotic particle yet to be discovered or just a property of gravitational force, which we have not been able to understand? We do not know. We know what dark matter is not. Lost planets, dead stars, dark nebulae are not considered dark matter because they would absorb, emit or reflect light. It is also not made up of subatomic particles like neutrinos, which are known not to interact with light despite being abundantly present, as they are as light as electrons and incapable of exerting substantial gravitational force. Maybe the answer lies somewhere in between and we need to marry the two proposed theories to solve this cosmic puzzle. Understanding dark matter is one of the biggest questions in science today, and some of the greatest minds in astrophysics are trying to decipher it.
Computer simulations that also create a digital universe provide compelling evidence for the existence of the elusive dark matter. In the simulations, we start from large quantities of dark matter that were born during the Big Bang, we let it evolve thanks to the gravity which makes it stick to each other, transforming the perfectly homogeneous space into clusters of dark matter. These eventually become the home of galaxies and also develop filamentous structures connecting different galaxies creating what is called a cosmic web. The simulations make predictions about how galaxy clusters are in the universe, and the results match the observable data perfectly. Observing the cosmic web and the gravitational lens is like possessing the DNA of dark matter, thus making its presence irrefutable. Physicists around the world have built sensitive detectors deep in the earth and even in space to pick up traces of dark matter. In India, the Jaduguda laboratory located in an abandoned uranium mine in Jharkhand is dedicated to unraveling the mysteries of dark matter. We’re even trying to make dark matter in the lab, with CERN in Switzerland the research flagship, by smashing subatomic particles in the Large Hadron Collider. We must continue to seek and think harder to understand dark matter in order to unlock a whole new understanding of everything and everyone in our known universe.
Einstein taught us that everything, including light, is influenced by gravity. As light from a distant galaxy passes through a cluster of galaxies en route to Earth, it bends, creating multiple images of the background galaxy, an effect called gravitational lensing by astronomers. Here, the dark matter in the galaxy cluster acts like a lens. If there hadn’t been dark matter, there wouldn’t have been an observable gravitational lens. The most massive related objects in the universe are galaxy clusters, which include hundreds to thousands of galaxies. They too are surrounded by dark matter which is responsible for the massive gas velocity in the cluster, providing further evidence for the mysterious matter.
(Author is PGT – Physics at Shiv Nadar School, Noida) Dark matter may have wrapped itself in a black shroud, but it will not be able to hide for eternity from the indomitable human spirit and relentless curiosity.
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