Astronomy Jargon 101: Dark Matter

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll feel mysterious about today’s topic: dark matter!

Something funny is going on in the universe. Way back in the 1930’s astronomer Fritz Zwicky was studying the motions of galaxies inside the Coma galaxy cluster. He found that those galaxies were moving around way too quickly. Once he added up all their mutual gravitational attraction, their incredible speeds should have ripped apart the Coma cluster ages ago. But there the cluster was, sitting there. He figured that there must be some sort of matter hidden from his observations, providing enough gravitational pull to keep the cluster together.

He named the stuff “dark matter” and moved on to other subjects. The topic was largely ignored by the astronomical community.

In the 1970’s astronomer Vera Rubin was examining the motions of stars within the Andromeda galaxy. She found that the stars were whipping around in their orbits way too quickly. That galaxy, and the dozens of others that she studied, should have torn itself apart, unless there was some hidden kind of matter to hold everything together.

Dark matter came back.

Since then, more evidence has mounted. The mass in galaxy clusters can bend the path of light from background objects, and astronomers can use the amount of bending to calculate the mass of the clusters. Those calculations reveal that the true mass of the clusters is at least 5 times that of what we can see.

Cosmologists studying the early universe can map the cosmic microwave background, the leftover light pattern from when the cosmos was only 380,000 years old. Those observations tell us that there was a form of dark matter knocking around back then.

Calculations of the formation of elements in the very early universe successfully predict the abundances of hydrogen and helium…and tell us that the amount of normal matter isn’t very much.

Everywhere we look, we see signs for dark matter. No other theory can account for the wide variety of observations that point to its existence. As best we can tell, it’s some new kind of particle, previously unknown to the standard model of particle physics, that simply doesn’t interact with light. Light passes through it – it doesn’t reflect, refract, glow, or absorb. A better name would be “invisible matter” but I guess that doesn’t sound as cool.

The hunt is on to find direct evidence for dark matter. So far, those searches have turned up empty. But I suppose if it were easier to see dark matter, it wouldn’t be very dark after all.