Here is a question you don’t think about everyday: where did the stuff you’re made of come from?

We know we are made of cells, and cells are made of molecules, and molecules are made of different atoms. But where do the atoms come from? The ground? Food?

Well, food is also made of living stuff, or at least stuff that lived before, so the question is the same there. The answer, if you don’t know it, may surprise you: all the atoms that are part of our bodies — iron, calcium, oxygen, etc. — and, by the way, that are part of everything else we see around us, come from stars. We are literally, and lyrically, animated bundles of stardust. This connects us deeply with the universe, in more ways than we can imagine.

Science is a way of making sense of the world. It is a narrative, a story in a way, that people have been creating for quite some time. The key difference between the story science tells and other stories is that science tells a story that can be verified, that deals with facts. For science to be a credible story, it needs to be verifiable. And no wonder, given that its main topic is “reality.” Now, reality is a pretty loaded word, as it can mean different things for different people. For scientists, reality has to do with what we can see and measure of the world, with and without instruments. Telescopes, microscopes, detectors of all kinds are “reality amplifiers,” opening windows into parts of nature that are too small or too far for us to see. We search for the invisible and see plenty of it.

We search for the invisible and see plenty of it.

Thanks to our amazing instruments the modern picture of the universe is quite shocking. The atoms that we are made of, and that we see in stars and in planets, are only a tiny contribution to what fills the cosmos: five percent. This is a strange picture of reality, where the common kind of matter is the absolute minority.

How can we know about the rest if it doesn’t shine? First, we must explain that “shining” is not the only way to see something. Things radiate, that is, they emit radiation in the form of electromagnetic waves, even if human eyes can’t see most of these waves. When you turn on a lamp, the light you see is coming from vibrations in the atoms that make up the filament. We can say that light is how atoms sweat. They may do so in the visible, and we can see it, or in invisible kinds of “light,” such as infrared, or ultraviolet. So, a modern astronomer sees the objects in the cosmos by collecting all kinds of light, from the visible and the infrared to more exotic ones, like x-rays and gamma rays.

Even after looking for all these kinds of radiation, matter made of atoms comes to five percent of the total. The other 95 percent are divided into two “ingredients”: dark matter and dark energy. The British author Philip Pullman, in his amazing trilogy His Dark Materials, explores the existence of dark matter fictionally (“dust”). At this point fiction is not a bad way to explain dark matter, given that we don’t really know what it is. We know it’s there because its mass pulls on objects we can see, especially galaxies. Galaxies move at huge speeds, and their motions are the results of two main effects: First, they drift away due to the expansion of the universe, as corks carried by a river. Second, they may attract one another and gain local deviations from this “cosmic flow.” To “see” dark matter, astronomers measure the speeds of galaxies and find that they are too big to be caused only by the stuff we see. The amazing thing is that this dark matter (“dark” from not being able to make light of any kind) is five times more abundant than ordinary matter, making up for about 25% of the stuff in the universe. An invisible cloak of dark matter particles surrounds us — our galaxy. Some may be going through you right now and you wouldn’t know.

We still have 70% left over. This is the even bigger surprise. In 1998, two groups of astronomers discovered that stars in faraway galaxies were drifting away from us way too fast; as if the universe were in a hurry to grow even bigger. The speeds at which these galaxies were moving away started accelerating some five billion years ago, about the same time the sun and the planets formed. No one expected this one. What could cause this kind of “repulsive gravity”? Well, we have candidates, possible explanations from pretty exotic physics. But we still don’t know the answer. What we do know is that to explain the observed cosmic acceleration, this “dark energy” needs to contribute the leftover 70% of stuff filling the universe. According to modern cosmology, the part of physics that studies the universe, we live surrounded by dark and mysterious stuffs.

This may sound weird, given that science is always learning more about the world. But if you think about how science actually works, it makes perfect sense. The best way to see this is to imagine what I call the Island of Knowledge. If everything that we know of the world would fit into an island, this island would grow as we learn more. Sometimes it may also shrink — when an idea we think is true is actually false. (This does happen, more often than you imagine.) On average, however, the island grows. Now, as with every good island, an ocean also surrounds this one. It is the ocean of the unknown, what we still don’t know of the world.

You would think that as the island grows, this ocean of the unknown would shrink away. Some people believe that one day the island will take over and we will know everything there is to know about nature. That, I argue, is an impossible dream. As the island of knowledge grows, so do the shores of our ignorance, the boundary between the known and the unknown. In practice this means that the more we know of the world, the more questions we can ask about it. Think, for example, of biology before and after the microscope. Before it was invented in the late seventeenth century, people had a very simplistic view of life: living things were big and moved about. After the microscope, a whole new world of what life is and how small it can get opened up. New questions could be asked that no one could have imagined before.

New instruments often reveal new universes for us to explore. The ocean of the unknown grows with the Island of Knowledge. After all, how could we possibly ever know all the questions we can ask about the world? Happily, we are surrounded by mystery, by the unknown. And that’s what makes science so exciting. It’s about what we don’t know of the world, the unseen stuff, big and small. What’s important is not to know all the answers but to keep on asking. As Tom Stoppard wrote in his play Arcadia, “It’s the wanting to know that makes us matter.”