Have you ever played with big what ifs? What if you were born 5,000 years in the future? What if the continents sat at different latitudes? What if the Sun were 10 percent larger? Astrophysicist Risa Wechsler does something similar for a living, only at cosmic scale. She builds model universes inside computers, each with different starting conditions and ingredients, then compares them to the sky we can measure. Those comparisons reveal a strange truth about our cosmos. Most of the matter in the universe is invisible to us, yet it shapes everything we see. That unseen stuff, dark matter, makes up about 85 percent of the mass in the universe, and without it, there would be no galaxies, no Sun, and no us.
Here is the full talk that inspired this guide. It is a great watch before you dive in.
What Is Dark Matter? The Unseen Influence
Dark matter sounds mysterious, but the idea is simple. It is matter that does not emit or absorb light. Telescopes cannot see it directly, yet it pulls on the matter we can see. In that sense, it acts like an invisible scaffold for the universe.
- Visible matter, like stars, planets, and glowing gas, is only about 15 percent of the total mass.
- Dark matter is the other 85 percent. It is everywhere, likely passing through you right now.
A helpful analogy is a picture of Earth taken at night. You see city lights, but you cannot see mountains, forests, or people. You can guess what exists by reading the patterns. Dark matter works the same way. We infer its presence from how it tugs on stars and bends light on its way to us.
If you want a clear primer, this short explainer gives a friendly overview of the basics: NASA’s dark matter overview. For a deeper reference, explore the background and evidence in Wikipedia’s dark matter article.
Why we know it is there
- Gravity exposes it. Dark matter changes the motion of stars in galaxies and the way galaxies move in clusters.
- Light warps around it. As light travels to us, the gravity of all matter, including dark matter, bends its path. We can measure that bending.
- We have not seen it directly yet. But its effects are consistent across many scales, from the smallest galaxies to the entire sky.
Dark matter also feels personal. The Earth spins on its axis, orbits the Sun, and the Sun moves through our galaxy at about 500,000 miles per hour. That means dark matter particles are likely streaming through your body all the time. They barely interact with ordinary matter, so they slip through without a bump.
How Galaxies Form: Dark Matter’s Crucial Role
If we want to understand why we exist, we need to understand how galaxies form. That story starts at the very beginning.
From the Big Bang to the first structures
Right after the Big Bang, the universe was expanding fast. Quantum mechanics says that particles can pop in and out of existence. In the earliest fraction of a second, expansion happened so quickly that newly created matter did not have time to annihilate. That process set the stage for all the matter in the universe, both normal matter and dark matter.
About 400,000 years later, the universe had cooled enough for protons and electrons to form hydrogen. Space was hot and dense, and very smooth, but not perfectly smooth. There were tiny spots that were a bit denser and hotter than others. A space mission called Planck mapped those tiny temperature differences across the sky. Each warmer or cooler patch in that map is a clue to where there was slightly more or less mass.
Gravity amplified those small differences over the next 13.8 billion years. The universe kept expanding, so the average density kept dropping, but the denser spots pulled in more matter. Those regions grew into the skeleton of the universe we see today.
From smooth to clumpy: what simulations show
Computer simulations bring this growth to life. Start with a nearly smooth universe, with a few regions that have slightly more material. Gravity does the rest. Mass flows into those regions, deepening their gravity wells. Hydrogen gas follows the pull, then cools and collapses. Stars form. Small galaxies appear first. Over billions of years, those small galaxies crash together and merge, building larger galaxies like the Milky Way.
A simple timeline helps:
- Early universe: almost smooth, with tiny over-densities.
- Gravity pulls matter into those denser spots.
- Gas cools, stars ignite, small galaxies take shape.
- Small galaxies merge, forming big galaxies over time.
Now imagine two side-by-side universes. One has dark matter, the other does not. In the universe with dark matter, clumps grow fast, gas collects, and stars form once regions pass about a million solar masses. In the universe without dark matter, nothing gets dense enough. Clumps stay small. Star formation stalls. No Milky Way, no Sun, no planets, no life. That is how central dark matter is to cosmic structure.
The Mystery Particle: Guesses and Hunts
We have strong evidence dark matter exists, but we still do not know what it is. The leading idea is that it is a new kind of particle. It feels gravity, but it does not interact with light. It also barely interacts with normal matter.
Here are a few traits researchers consider:
- It passes through the Earth, and us, without stopping.
- Its mass could be tiny like a subatomic particle, or extremely heavy. Ideas span a huge range.
- It seems to avoid interacting with normal matter in any obvious way.
So how do we find it? Scientists are running several types of searches at once.
- Underground detectors. Deep in mines, sensitive instruments wait for a rare collision between a dark matter particle and atoms in a detector. If a collision happens, it can leave a tiny trace of energy.
- Telescopes in the sky. If dark matter particles can smash into each other and create high-energy light, telescopes that see gamma rays might spot the glow.
- Particle colliders. Experiments at the Large Hadron Collider in Switzerland try to make dark matter by smashing known particles together at high energy. If energy goes missing in a way that matches the predictions, that could be a sign.
So far, these approaches have not found the particle. That is still progress. Experiments rule out many early ideas and help narrow the search. Scientists keep raising the sensitivity and trying new designs.
Clues from Galaxies: Mapping the Invisible
Even as labs hunt for particles on Earth, the sky offers rich clues. Galaxies tell us a lot about what dark matter does.
Large-scale maps: bending light to see the unseen
We can make maps of matter in the universe by measuring how gravity bends light from faraway galaxies. This effect, called gravitational lensing, lets us infer where matter sits between us and those galaxies, even if that matter is dark.
Risa Wechsler and collaborators worked on a project called the Dark Energy Survey. It measured the positions and shapes of about 100 million galaxies across one eighth of the sky, creating the largest map of its kind at the time. By analyzing how galaxy shapes are subtly stretched, the survey built a map of matter in that region, including dark matter.
These maps help us:
- See how much dark matter there is.
- Find where it collects.
- Track how structure changes over time.
- Test predictions from simulations about galaxy growth and motion.
This mapping approach is one reason the case for dark matter is so strong. The same invisible scaffolding that shapes galaxies in simulations shows up in the way light reaches us.
Tiny galaxies, big hints
Small galaxies are surprisingly powerful clues. Simulations show that the number of small clumps around a Milky Way-like galaxy depends on how fast dark matter particles move. If the particles are fast, they escape small gravity wells. Fewer tiny clumps form. If they are slower, more small clumps survive.
We can check that with observations. In the southern sky, you can see two small galaxies with the naked eye, the Large Magellanic Cloud and the Small Magellanic Cloud. In the last decade, sky surveys found many more tiny companions around the Milky Way. Some of these are so small they have a few hundred stars, compared to the few hundred billion stars in our galaxy.
A few highlights:
- Surveys like the Dark Energy Survey uncovered new ultra-faint dwarf galaxies.
- We now know of about 60 tiny galaxies orbiting the Milky Way.
- Their existence shows dark matter cannot be moving very fast, and it does not do much when it bumps into normal matter.
These small systems are hard to spot, which makes each detection a big deal. They sharpen our picture of how dark matter behaves. They also put pressure on theories, since not all dark matter models would allow so many small galaxies to survive.
If you want to compare how different ideas line up with the evidence, this background page is a useful reference: Dark matter on Wikipedia. For a high-level, public-friendly overview with images and graphics, try NASA’s dark matter resource.
Looking Ahead: Why This Search Feels Close to Home
The next few years look exciting. New surveys will make even more precise maps of the sky. Better detectors will push the limits in underground labs. Particle experiments will keep looking for signs of new physics. Each step tightens the net.
We already have clear evidence for dark matter across scales. It shapes tiny galaxies and the web of structure across the entire universe. Even if we do not catch the particle soon, we will learn more about what dark matter can and cannot do. That still moves us forward.
If this topic grabbed you, browse more talks and ideas on TED.com, and subscribe to the TED channel for new science talks on YouTube through TED’s YouTube channel.
Conclusion
Dark matter makes up about 85 percent of the mass in the universe. It does not shine, yet it quietly sculpts everything from small galaxies to giant clusters. It is probably streaming through you right now. From the first hot moments after the Big Bang to the rise of the Milky Way, dark matter’s role is hard to overstate. Without it, stars would have struggled to form, and life as we know it would not exist.
Keep an eye on new sky maps and lab searches. Each result, even a null result, sharpens the picture. What we learn next could reshape physics, and deepen our understanding of our place in the universe. Curious minds are welcome here.
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