25 Days of Particles: Day 21

Dark Matter

Classification: Unknown
Fundamental: yes (we think)
Mass: Unknown
Interactions: Gravity and maybe weak
Spin: Unknown
Lifetime: Unknown

If the point of particle physics is to answer the question "What is the universe made of?", this next "particle" is embarrassing evidence that we aren't doing all that great a job.

The story begins in 1933, with Fritz Zwicky and galaxy rotational curves. He was measuring the speed of stars in galaxies and found they were moving very quickly. It takes a lot of mass to produce the gravity necessary to hold quickly moving objects, so Zwicky used the measured light from the stars closer to the center of the galaxy to estimate how much mass was holding the fast stars into the galaxy. There was not enough visible mass to hold the outer stars into the galaxy, not at the speeds they were traveling. There should have been stars flying out of galaxies at high speeds, instead of obeying some mysterious force that kept them in their orbits.

The foremost theory was that there some additional matter that wasn't giving off light, and that matter was producing the extra gravity needed. It was called appropriately dark matter, so that everyone knew which mystery was being referred to. There were also ideas that the effect could be due to our theory of gravity being wrong or something like that.

However, over the next eighty years, evidence for dark matter piled up, from gravitational lensing and the collisions of galaxy clusters and the cosmic microwave background radiation. Our theories of gravity seem to still be working, just with some matter that we can't see.

Of course, we already know that there are lots of things in the universe that don't give off light. Planets and asteriods and other big rocky things don't give off light, and at first those things were thought to comprise dark matter. They were called Massive Astrophysical Compact Halo Objects, or MACHOS, and surveys began searching for them. They found a few, but not enough to explain dark matter. The new idea that emerged was that the dark matter was a particle sitting around in the universe not doing much. These particles were dubbed Weakly Interacting Massive Particles, or WIMPS.

This sounds kind of like the behavior of neutrinos, but we can measure the neutrinos and there aren't enough of them either. Our best estimates of the amount of neutrinos out there says that there is about equal amounts of neutrinos and baryonic matter, increasing the amount of matter we are aware of by a factor of two. We need to increase by about a factor of six to explain dark matter.

See why this is somewhat embarrassing? We know a whole lot about the matter that makes up one-sixth of the stuff in the universe. Regarding the other five-sixths, we're looking for it. Of course, we have plenty of evidence for how it doesn't directly show up, but there are still hopes that very, very sensitive experiments can detect dark matter particles when they collide from the earth, or that colliders can make dark matter particles and find evidence of their missing energy, much in the same way neutrinos were discovered.