First posted 24 Nov 2003. To check on any updates to our knowledge, you can visit Nasa.
We know our universe pretty well, right? Galaxies contain stars, around which orbit planets. The odd comet or asteroid zips around. And there is dust, like what gives Saturn those cool rings. We can see these things all around us, reaching to the edge of the visible universe.
All of this stuff is made of matter, which most of know is made up of protons, neutrons and electrons. Some of us know that protons and neutrons are actually constructed of more fundamental particles called quarks and that there is a small host of other fundamental particles out there. We have a pretty good idea of the mass, electric charge and spin of each of these particles.
Sounds good. Well imagine my surprise when I learned that all the particles we know of make up only about 5% of the estimated mass-energy content of the universe, and that its very existence is a fluke!
What's the matter?
Cosmologists are able to measure the orbital motion of galaxies within clusters, and of stars within galaxies. The velocities they have measured are greater than can be attributed to the gravitational field generated by the mass of visible matter. When we calculate the mass that must be 'out there' to account for the observed velocities, we learn that visible matter makes up only 15% of total matter in the universe.
The rest is called Dark Matter, and we know precious little about it. We reckon that it is made up of Weakly Interacting Massive Particles (WIMPs!), that it formed into structures as the universe cooled after the Big Bang, and that the visible galaxies we see formed around/within these structures.
The Dark Side, continued
But wait, there's more. It turns out that matter (including Dark Matter), makes up only about a third of the Cosmological balance sheet. The remainder, about which we know next to nothing, is called Dark Energy. How do we know it's there? Well, we've wondered for some time whether the universe is destined to go on expanding or to collapse upon itself in a Big Crunch. Scientists have measured very closely and learned that the rate at which the universe is expanding is actually accelerating. Well, first, this means there's no Big Crunch in our future. But second, it leads us to ask what force is counteracting gravity to cause this acceleration. Dark Energy is the place holder for this variable.
That bursts our bubble a bit, huh? Well, we should consider ourselves lucky - really. It's a fluke that there is any visible matter at all. As Einstein's famous equation shows, mass and energy can be converted between one another. This was happening at a fantastic rate in the earliest moments after the Big Bang. Particles burst into existence, accompanied by their mirror-cousins, anti-particles. Usually, particles and anti-particles ended up slamming into one another and annihilating to form energy again. But one time in a billion a particle was created without an anti-particle. This slight asymmetry led to the existence of the visible matter that we observe (and are made of) today. We are but an asymmetric excess!
Here's lookin' at you kid. You're one in a billion. But you've got a way to go before you really understand your universe.