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Particle physicists have constructed a family of 12 fundamental particles, divided into two groups of six: quarks and leptons. "Up" and "down" quarks (the names are strictly arbitrary; there is no up or down orientation peculiar to either particle) are the building blocks of protons and neutrons. One of the leptons is the familiar electron. Also in the lepton class are wispy, nearly massless neutrinos that interact only very weakly with other particles. Elusive as they are, neutrinos are abundant in the universe and may be candidates for the missing dark matter.
At 10^-12 second, the weak and electromagnetic forces separated, leaving us with the four separate forces we know today. The weak force, which governs radioactive decay, acts only over very short distances. The more familiar electromagnetic force governs electricity, magnetism, and the propagation of electromagnetic radiation such as visible light.
During the Grand Unification Epoch, quarks and leptons and their corresponding anti-matter particles were constantly colliding and annihilating each other with a release of energy; two colliding photons could likewise create new matter and antimatter. What's more, quarks could decay into leptons, and vice versa. Matter, anti-matter, and radiation existed in nearly equal amounts.
There's hardly any antimatter left today (and a good thing, too, or we wouldn't exist: everything would have been annihilated long ago!). What happened to it?
Just before the end of the Grand Unification Epoch, there was a very slight excess--perhaps one in a billion--of matter over antimatter. Quarks could decay, or be created, without the corresponding decay or creation of antiquarks. As the universe continued expanding and cooling and the strong force separated from the electroweak force (which would later separate into the electromagnetic and weak forces), quarks and antiquarks condensed into hadrons.
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When the universe was about one second old, the universe cooled to the point that electron-antielectron pairs could no longer be created. The result was another mass annihilation, creating even more photons and leaving behind a few electrons.
The weak force was losing its effectiveness to mediate interactions between neutrinos, antineutrinos, and the other particles. Neutrinos no longer interacted with other particles or even antineutrinos, thereby escaping annihilation. They began--and continue today--to drift through space in enormous numbers; some scientists believe they might account for a significant portion of the missing dark matter.
Now the stage is set for the formation of atomic nuclei--from which all the matter we see in the universe today originates.
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