It appears that we have misplaced most of the universe.
Or, more accurately, scientists who are in a position to know about these things have been unable to put their hands on it. This hints at a carelessness that is worrisome in people who build underground racetracks where they crash sub-atomic particles into each other in order to create new particles or antimatter. Antimatter?! On our own personal planet?! Is this a good idea? Don't matter and antimatter particles annihilate each other when they come into contact? Maybe I've watched too many Star Trek episodes, but this makes me nervous. I wish they'd go race cars or something.
At any rate, scientists began to suspect that a lot of the universe was missing when they came across a discrepancy between the velocity and the calculated mass of galaxies and galactic clusters. Individual galaxies were travelling faster than then-current theories said they should be. In fact, they were travelling so fast that the clusters should have been flying apart, as if they were playing a cosmic game of crack-the-whip. Clearly something was holding them together, and scientists decided that the 'glue' was additional galactic mass that was unobservable with current techniques. They called the unobserved mass Dark Matter.
What is 'mass'? People often use the term interchangeably with 'weight', but they are in fact different. Mass is how much stuff a body contains. Weight is the effect of gravity on that stuff. For example, an astronaut working in orbit at the Space Station is weightless but he still has mass. To be precise, Weight Watchers should really be called Mass Watchers. You want to get pedantic with a roomful of hungry people? Go right ahead.
Of MACHOs and WIMPs
Where was I? Oh, yes. Once scientists decided that a large portion of the universe was made up of this Dark Matter, they were off and running, trying to figure out what it was. Several possible candidates emerged, with the two main categories being classified as MACHOs and WIMPs. These tongue-in-cheek acronyms make it easy to remember which type is which.
The theory of MACHOs (Massive Compact Halo Objects) says that the missing matter can be found in the 'halos' surrounding galaxies. This would include burned-out dark stars, stray planets, black holes, and other large, heavy, but dark objects. These are made up of baryonic, or ordinary, matter, the same stuff you and I are made of. The astronomers hunt for these guys.
The other leading candidates are collectively known as weakly interacting massive particles, or WIMPs. These are exotic, heavy particles that remain almost entirely unobserved in experiments. The physicists hunt for WIMPs. No witticisms, please, from the astronomers in the crowd.
If this weren't confusing enough, scientists also categorize dark matter candidates as 'hot' or 'cold', depending on the speed at which the particle was moving at the time when galaxies should have started to form. One such 'hot' particle is the neutrino, which is the first serious dark matter candidate actually known to exist.
These scientists! Comedians, the lot of them. The universe is no slouch in the comedy department either. It has managed to hide most of itself where we can't see it, right under our baryonic noses.
Currently the WIMPs appear to be winning the battle, in that they probably make up the majority of dark matter. Some MACHOs have been found, but they are relatively few in number and can account for only a small part of the universe's missing mass. The meek are indeed inheriting the earth, and all the rest of the real estate as far as the eye can (or can't) see.
The amount of mass in the universe controls the fate of the universe, which scientists hypothesize has been expanding ever since the Big Bang. They compare the average density of the universe with the average density required to halt the expansion of the universe (called the critical density). The ratio of the average density to the critical density is known as Omega. The value of Omega tells us if the universe will:
- Recollapse if Omega is greater than one.
- Expand forever but gradually slow down and stop at infinity if Omega is equal to one.
- Expand forever if Omega is smaller than one.
So scientists are eager to determine the amount of dark matter in the universe to get a hint of coming attractions. Experiments continue to refine their estimates of the value of Omega, which tend to hover tantalisingly around one. This is compelling stuff, even if you're not planning to hang around long enough to witness a possible Big Crunch.
There is also the admittedly remote possibility that we'll discover some new and exotic particle that will throw into doubt a lot of what we think we know about the universe. Maybe there was no Big Bang, for example. This is always exciting, at least for those whose careers haven't just gone up in smoke.
Find Out More
Had enough? If not, here are some places to find out more about dark matter, galaxies, and whatnot:
BBC's Dark Matter Page
The Edited Hitchhiker's Guide Entry on Galaxies
An Introduction to Extragalactic Astronomy