Dark Matter in the Universe: How to Understand it in Layman’s Terms

This story came to the screen in 2024, as part of the Nova series. It was called Decoding the Universe: Cosmos. I had no idea what the story was about, but had always been interested in astrophysics. My four-year PhD was in space science: how cosmic rays from the sun travel to the earth. I spent the following ten years doing research in this field. Then I switched horses to a petroleum engineering job where I mostly did research on oil and gas production.

Back to astrophysics. The terms dark matter and dark energy have crossed my path many times, but I never understood what they were until I watched an enticing PBS story in the Nova series. This was just last week. Here is a simple explanation of dark matter as conveyed by the Nova story.

Dark Matter.

Discovered when: 1933, confirmed in 1970s.
By: Fritz Zwicky in 1933, then Vera Rubin in 1970s.
How confirmed: rotation of stars in the Andromeda galaxy.

Vera Rubin was an observational astronomer, who loved to collect data from telescopes, and then analyze the data. She was restricted jobwise because astronomy was regarded as a male institution. But then she was accepted at the new Kitt Peak Observatory in Arizona, an hour or two from Tucson. I visited Kitt Peak in 1981.

The Andromeda galaxy is the nearest galaxy to our own Milky Way galaxy. It’s a spiral galaxy containing two trillion stars. These stars rotate about the center of the galaxy kind of like planets rotate around our sun. The center of Andromeda is a gigantic black hole, named M31, equivalent to 120 million sun masses.

In our solar system, inner planets rotate around the sun faster than outer planets. It’s a law of gravity because the gravity force of the sun falls away with distance from the sun. Astronomers expected the same law to apply to stars rotating around the center of Andromeda.

What Vera Rubin found was that outer stars were moving just as fast as inner stars in Andromeda. This didn’t make sense because these fast-moving stars should shoot off into deep space, like when you whirl a stone on a string around your head then suddenly release the string.

Something was stopping the outer stars from flying away. The explanation by Rubin and others was that the Andromeda galaxy must have a halo of invisible material whose mass was holding the outer stars in their orbits. This was called dark matter because it was invisible.

But Rubin’s measurements gave the same result for many other galaxies. Astrophysicists now think that dark matter is arranged in a vast filamentary network throughout the universe. But the shocker is, there is five times more dark matter than visible matter in the universe.

What could be the source of dark matter?

There are three possibilities:
MACHOS: this stands for massive compact halo objects. These could be black holes left over from early days of the big-bang, and sprinkled throughout the universe. The kicker is for them to be invisible, these tiny black holes must have the mass of an asteroid but only be the size of an atom. Wow!

Axions: these would be miniscule particles theorized by physicists.

WIMPS: this stands for Weakly Interacting Massive Particles. A deep gold mine has been set up in the Black Hills of South Dakota to try to capture a WIMP. At the center of the LZ Detector is a big bucket that contains seven tons of liquid xenon, which has to be very cold. The xenon is very sensitive to an impact by a cosmic ray particle or a WIMP. The mine is a mile deep which prevents penetration by any cosmic ray particles.

There are other experiments looking for WIMPs, but so far none have been found and verified. This is motivating some scientists to lean more toward Axions as the source of dark matter.

The past 50 years have been humbling. During this time, some of the smartest scientists in the world have investigated and defined amazing objects like neutron stars and black holes, and what they do in the universe. In addition, they have proven that the universe is accelerating outwards (due to dark energy).

But dark matter and dark energy are invisible. They neither emit radiation, nor interact with radiation. And none of the scientists can explain what they are made of. This is even more humbling because dark matter and dark energy together make up 95% of the mass-energy content of the universe. Galaxies, stars, and planets make up only 5% of the universe.
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We are into May 2026, and the weather is warming…. Mostly in the 80s F in Albuquerque this week. The city lies at 5,000 feet altitude, so the summers aren’t too hot.
The Gray Nomad.
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For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen.
[Romans, chapter 1]