Dark matter vs. antimatter: Two cosmic mysteries that sound alike but aren’t

Space News explained two concepts in physics that often get confused due to their similar names. Dark matter and antimatter are not the same. Scientists study both to understand how the universe works.

One is known through theory and experiments on Earth. The other is known through its effects in space. The article answers a question sent by a 13-year-old reader from India and is part of a series that explains science topics for young readers.

The story employs simple examples to help the reader understand the behavior of matter, antimatter and dark matter.

Regular matter is the one that makes up atoms, stars, planets and people. Antimatter is the one which is made of particles that correspond to the particles of regular matter, but the electric charge of the antiparticles is just the opposite of that of the particles of matter.

The first thing that happens when matter and antimatter meet is that they become energy.

Dark matter is different. It does not emit light and does not interact with atoms in the same way as it is currently understood, but it determines how objects move in space through gravity.

Scientists developed these concepts in entirely different ways.

The existence of antimatter was revealed by a theoretical framework and later found by experiments. Scientists introduced the concept of dark matter after they discovered that galaxies move at a speed that is twice that which can be explained by visible matter alone.

What scientists know is based on the measurements and observations that have been made. What they don't know is the precise composition of dark matter. The explanation is based on unambiguous facts, simple language and straightforward examples.

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Antimatter and how scientists study it

Antimatter is composed of particles that mirror the properties of regular matter particles. An electron has an antimatter partner called a positron. A proton has an antiproton.

These particles can form anti-atoms. When antimatter interacts with regular matter, both are annihilated and energy is released. This process is called annihilation.

Antimatter is rare in the universe today. Small amounts can form through natural processes. Some atoms, such as potassium, can decay and produce positrons.

This happens in foods like bananas. The amount is very small and does not pose a health risk. As the article states, “the amount is too small to affect your health.”

Scientists discovered antimatter nearly 100 years ago. Since then, they have made and stored it in laboratories. They know its mass, charge, and behavior.

Antimatter is also used in medicine. PET scans work by tracking the light created when antimatter interacts with matter inside the body. These scans enable doctors to visualize organs and tissues.

Our early universe likely had similar amounts of matter and antimatter. Most of it destroyed itself. A small imbalance left more matter behind. This remaining matter formed galaxies, stars, and planets.

A key question remains: “If matter and antimatter annihilate each other when they touch, how is it possible that there is now so much more matter than antimatter in the universe?”
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Dark matter and what astronomers observe

Dark matter is not seen directly. Scientists know it exists because of how objects move in space. It affects motion through gravity. It neither emits light, absorbs light nor reflects light. Because of this, telescopes cannot see it.

In the 1970s, Vera Rubin measured the speeds at which stars orbit around the centers of galaxies. She found that stars far from the center move faster than expected.

According to the article, “there must be a sea of invisible ‘stuff’ holding everything together with their extra gravity.” This unseen mass was named “dark matter.”

Further studies have shown similar effects in many locations. Galaxies in clusters move in ways that visible matter alone cannot explain. Light from distant objects bends more than expected as it passes galaxies. These measurements support the idea that extra mass is present.

Scientists do not yet know what it is made of. Many experiments have tried to detect dark matter particles, but none have succeeded so far.

Nevertheless, measurements indicate that dark matter is abundant. Observations suggest there is about five times more dark matter than regular matter in the universe.
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