Vera Rubin's Essay: What Is Dark Matter?

In the 1960s and 1970s, astronomer Vera Rubin was observing spiral galaxies. She was interested in how they rotate, because you can learn a lot about a galaxy that way. Think about the Solar System. Back in the 1600s, Johannes Kepler figured out that the farther a planet is from the Sun, the slower it orbits. Isaac Newton put numbers to that, calculating the strength of the Sun's gravity, which means we could in turn get the Sun's mass.

Same with galaxies. If you can measure how they rotate, how rapidly gas clouds move in their orbits near the edge of the galaxy, for example, you can calculate the mass of the entire galaxy. Galaxies are so big that you can't physically see the nebulae move, but you can measure their Doppler shift, which gives you their velocity.

What Rubin expected to see was that the farther out from the center of the galaxy the gas cloud was, the slower it would be moving, just like more distant planets from the Sun move more
Everything gives off some kind of light, but more observations just kept supporting the existence of dark matter.

What is Dark Matter? (3:52)

So what is dark matter? That was the big question. Astronomers were methodical. They listed every single thing they could think of that dark matter could possibly be: cold gas, dust, dead stars, rogue planets, everything. Even weird subatomic particles that were predicted to exist in quantum mechanics theories but never seen before.

Then they thought of ways they could detect these objects. Cold gas would emit radio waves, for example. But everything they tried came up empty. One by one they crossed objects off the list, and eventually, everything made of normal matter-- atoms and molecules, protons, electrons, and neutrons-- was eliminated. All that was left on the list was that truly bizarre stuff, those screwy subatomic particles no one had ever seen