Being born at 11,975 feet above sea level in one of the world’s highest cities might naturally inspire someone to gaze upward and wonder about the vast universe above. For Indira Ocampo, however, it wasn’t just the altitude that sparked her curiosity—it was her insatiable drive to ask questions, seek answers and uncover the mysteries of the cosmos. This passion led her to pursue a degree in Physics, specializing in Astronomy, Astrophysics and Cosmology, with a clear mission: to contribute, even in a small way, to humanity’s understanding of the universe.
Now, as a PhD student on the prestigious INPhINIT programme at the Instituto de Física Teórica UAM/CSIC in Madrid, Indira continues to explore the frontiers of cosmic knowledge.
This fellowship includes a transversal skills training programme. It was in one of the webinars included in the programme, where Indira, putting into practice what she learned, wrote this wonderful article about one of the questions she asks herself most often. Don't miss her reflection:
The Universe is a vast and mysterious system stretching out into the depths of space and time. It has captivated human curiosity for millennia, leading us to consider profound questions such as: How was the universe created? What is its fate? How likely is it that life exists somewhere else? Somehow, whenever we believe we’re about to understand it, we realize how little we actually know.
All the stars, planets and galaxies comprise just 5% of the total content of the universe. The other 95% is made up of what we call dark matter and dark energy. We have deduced that they exist because of their influence on galaxies but their true nature remains unknown. Understanding dark matter and dark energy would represent a breakthrough, with the potential of completely changing our knowledge of the universe and acknowledging our place within its immensity. Additionally, in the same way that quantum physics has had transcendental applications in the technology we use nowadays, the discovery of these two phenomena could transform our capabilities.
You may be wondering how we know that dark matter and dark energy actually exist. Who observed them first? In the early 1900’s, around a century ago, we had some understanding of the physics behind the atom and how it was composed (for example, we understood nuclear physics to some extent) but we were barely aware of the universe. For instance, we didn’t even know that the Milky Way (whose name, by the way, comes from “γαλαξίας (galaxies)”, “Milky” in Greek) was a galaxy on its own. This information may seem obvious to us now but, for young scientists in the early 1920s, it was a revolutionary thought that contributed to the emergence of a whole new field of study: cosmology.
Upon discovering there’s an entire universe beyond our galaxy, we believed we understood everything. But nothing could be further from the truth. In 1933 a physicist named Fritz Zwicky wanted to measure the visible mass of the Coma Cluster of galaxies (a “cluster” is a group of thousands of galaxies) but he hit a wall when he noticed a discrepancy between theory and observation: the mass of the cluster was ten times greater than the mass estimated from its observed luminous sources (like stars, galaxies). He therefore suggested the existence of some form of dark (invisible) matter for the missing mass. Later he became known as the “father of dark matter”. His work is currently regarded as the beginning of a paradigm shift in astronomy, something that deeply affected our perception of the universe.
Zwicky wasn’t taken seriously for another 40 years when Vera Rubin, one of the first female scientists in the U.S. allowed to pursue a scientific career in astronomy, observed the second piece of evidence for the existence of dark matter. Again, the problem was a mismatch between theory and observational data.
To explain this, we need to take a step back. It turns out that almost every large galaxy has a supermassive black hole at its centre. A supermassive black hole is an incredibly dense and massive region in space with such strong gravity that it pulls everything nearby into it; even light is unable to escape. This pull of gravity should cause the stars near the centre to rotate faster than those on the perimeter. To illustrate this, imagine our solar system and the planets orbiting around our sun. Those planets closer to the centre, such as Mercury and Venus, take only 88 and 225 days to complete a rotation around the sun, compared to those that are much further away like Uranus or Neptune, which take 88 and 165 years respectively.
Rubin was studying the rotation of certain galaxies when she observed that the outer stars rotated as quickly as those near the centre, suggesting that something else in the galaxy was providing some sort of additional gravity and, again, it seemed to be outweighing the “normal matter” by a factor of 10.
The third and irrefutable piece of evidence for dark matter was the detection of gravitational lensing. A gravitational lens is any form of matter, for example, a star or a black hole, that bends the light from a distant object behind it. If we looked at a galaxy behind a massive object, its image would appear distorted because its light wouldn’t travel in a straight line. Instead, it would be deflected due to the gravitational lensing effect. It turns out that some distorted images of galaxies were observed but without any apparent cause for their distortion, indicating that this gravitational lens is made up of dark matter.
All such observations suggested that this type of matter doesn’t interact with light (we cannot see it) but it seems to interact with gravity (we can see its effects on other visible objects). It was undeniable that dark matter existed and, apparently, it makes up 80% of the total matter in the universe! These final conclusions were held for 50 years after Zwicky’s initial prediction and 5 years after his death.
The discovery of dark matter is an amazing story but some other scientific milestones were also happening in parallel. In 1929, Edwin Hubble pointed at the sky with his telescope and made a tremendous impact when his work revealed the existence of galaxies beyond our own. For the first time, we were able to understand how immense our universe is and appreciate the fact that we may not be alone (if the universe is so big, there must be life somewhere else, right?). Hubble continued to study galaxies and, to his surprise, he discovered another counterintuitive phenomenon. Galaxies are affected by the gravitational influence of other galaxies so they should get closer with time but Hubble observed the opposite: galaxies were receding from each other, consequently moving away from us. The most natural conclusion was that there must be some type of “force” that’s pushing galaxies apart.
But what’s this supposed to mean? Back in the day, scientists thought that the universe was static, meaning that the space between galaxies and clusters wasn’t supposed to be expanding or contracting. But Hubble’s discovery that galaxies are receding was evidence of the expansion of the universe caused by an “invisible force”.
The scientific community explored this phenomenon in more detail and, as a result, in the late 1990s the detection of something called supernovae helped to clarify a piece of the puzzle. A supernova is the explosion of a star and it can be easily recognized (depending on the type) because it’s one of the brightest events in the universe and occurs in every galaxy from time to time. Besides being a fascinating phenomenon, supernovae are also useful because they can help us understand how galaxies move and, through this motion, we can estimate the nature of the expansion of the universe. We realised that the universe was expanding but we believed this expansion was slowing down over time due to the gravitational attraction between galaxies.
Supernovae revealed the opposite; instead of slowing down, the expansion was speeding up. This means that, as you read this article, the universe is not only expanding but this expansion is getting faster by the second.
“Dark energy” is responsible for this expansion; i.e. some invisible source of energy that exists throughout the universe and pushes things apart, adding more queries to the book of unanswered questions in cosmology.
Now, you might be wondering “what about ordinary matter?” What role does it play in this cosmic narrative? Well, ordinary matter is what we can see and touch; we encounter it every day. It’s the material that makes up the world around us, such as the ground under our feet, the air we breathe and even ourselves.
Ordinary matter forms the foundation of our physical reality. Yet, surprisingly, despite its familiarity, ordinary matter constitutes only a fraction of the universe's total composition. In fact, it accounts for a mere 5% of the cosmic pie. Meanwhile, dark matter, the invisible substance that interacts with gravity but not with light, has a far more substantial share, making up approximately 26% of the content of the universe. Then there's dark energy, responsible for the universe's accelerated expansion, dominating the cosmic landscape with a shocking 69% of the total content.
Let me go back to where we started. Why do we appear to know just 5% of the universe? We honestly don’t know why. However, we went through several paradigm shifts throughout the 20th century. These shifts usually happened in periods of about 5 and 12 years, but 27 years have passed since the last breakthrough that led to the last fundamental change of perspective in cosmology. After nearly three decades, one can't help but wonder: how long will it take to unravel the true nature of dark matter and dark energy? Why can’t we see them? Will the universe keep on expanding forever and, if so, where will this expansion lead us? What are we missing? So many questions still need answers but extraordinary efforts are leading us to set up telescopes both in space and on Earth, with very complex and expensive instruments. We have managed to identify billions of galaxies (when 80 years ago we knew only about the Milky Way!) and we’re developing sophisticated techniques to analyze the data collected. Sooner or later human cleverness and creativity will enable us to put together the pieces of this puzzle. However, perhaps then we’ll come to the conclusion that we actually know even less than we think – but that’s why we do science!
Like Indira, you too can pursue answers to the questions that fascinate your mind. How? By applying for a doctoral INPhINIT fellowship to carry out your doctoral studies in Spain or Portugal. The call for applications is open until 23 January 2025 for Incoming and 18 February 2025 for Retaining Call. Check out the requirements here and take your first step toward a future brimming with possibilities. The universe is waiting to see what you’ll discover and contribute!
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