What is Dark Matter?

It's great to dive into dark matter with you! Before we jump in, I'd love to know a little bit about what sparked your interest today and what kind of background you have with physics.

I'm just curious and have no background in physics!

Curiosity is the best reason to explore the cosmos! Since you're coming in fresh, let's start with a helpful picture.

Imagine going to a wind-blown beach on a dark night. You can't see the air around you, but when you watch the leaves or sand blowing across the ground, you know the air is definitely there because of how it pushes things around.

That is essentially what is. It's a mysterious form of mass scattered throughout space that we cannot see, but we know it's present because we can feel its heavy gravitational tug pulling on visible stars and galaxies.

What makes dark matter so strange compared to —the regular matter that forms planets, stars, trees, and your body—is light. Regular matter absorbs, reflects, or emits light. Dark matter, on the other hand, passes light right through without interacting with it at all.

I've put together a visual breakdown below to show how little of our universe is actually made of the regular stuff we see every day.

A pie chart showing the mass-energy budget of the universe, divided into 68% dark energy, 27% dark matter, and 5% ordinary matter.

As you can see, everything we've ever seen through telescopes—and every atom on Earth—is just 5% of the cosmos! About 27% is dark matter holding galaxies together, while the remaining 68% is dark energy, an entirely different mystery pushing space apart.

Continue: How do we know dark matter is really there?

Since we can't see dark matter directly, we know it's there because of the gravitational effects it has on visible objects. One of the clearest pieces of evidence comes from looking at how galaxies rotate.

In the 1970s, an astronomer named measured the speed of stars rotating around the centers of spiral galaxies.

She expected stars at the outer edges of galaxies to move much slower than those near the middle, just like planets in our solar system move slower the farther they are from the Sun's gravity.

A side-by-side comparison of galaxy rotation curves showing predicted Keplerian speed dropoff on the left and observed flat rotation speed with a dark matter halo on the right.

Instead, Rubin found that the outer stars were moving just as fast as the inner ones! The only way those edge stars wouldn't fly off into deep space is if the galaxy is embedded in a giant, invisible halo of mass pulling on them.

What is a black hole?

That's a fantastic question to ask! It's super easy to confuse the two because both sound like dense, heavy, invisible objects in space.

A is a specific place in space where normal matter has collapsed down to such an extreme density that its gravity prevents anything—even light—from escaping. Almost every large galaxy actually has a supermassive black hole sitting right at its center.

However, black holes can't explain dark matter. Black holes are point-like objects scattered here and there, whereas dark matter forms a massive, smooth cloud or halo spread out all around a galaxy. Plus, if there were enough black holes to make up all that missing mass, we'd constantly see them blocking background starlight or swallowing gas.

Now that we've cleared up rotation curves and black holes, another massive piece of evidence for dark matter is how it bends light, a phenomenon called gravitational lensing.