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Every space show I watch mentions that anti-matter used to exist, or still does and we just can't detect it. I think some shows even say we can create a small amount of anti-matter. It is not presented as an unproven conjecture like string theory, but rather as a fact.

In terms someone without a PhD might understand, what is the strongest and simplest evidence that anti-matter used to exist, or still does?

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Are you sure you don't mean dark matter? Anti-matter is well-known to exist. –  spencer nelson Jun 30 '11 at 20:11

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Anti-matter is produced all the time in the world's particle accelerators. It is also produced natrually during air shower cascades caused by cosmic ray particles and gamma-rays interacting in the atmosphere and when these same particles interact in particle detectors.

One of the simplest methods to show it exists is used by the cosmic ray detector on the Pamela satellite. In this detector, there is a target that the cosmic rays (and gamma rays) interact with. Then the particles pass through a strong magnetic field. Since the path of moving charged particles curves in the presense of a magnetic field, they can see where the particle entered and where it hit the final target which is a calorimeter that measures the energy.

Electrons come into the detector and curve one direction. Positrons, the positive anti-particle to the electron, enter the detector and curve the other direction. They have the same mass and energy but opposite charges and this is easily measured showing that positrons exist.

The same principle is used to detect them in the particle accelerators around the world.

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Indeed, antimatter is completely mundane to physicists, but only in individual particle quantities. You can't go down to the local physics supply store and buy a gram of antimatter. Or a milligram, or microgram, for that matter. The best that has been done is creating a few million molecules of antihydrogen (en.wikipedia.org/wiki/Antihydrogen). Which isn't much at all, even though that might sound like it. –  Andrew Jul 1 '11 at 2:46
    
...and it's a good thing you can't buy it! If 1 gram of antimatter came into contact with ordinary matter, it would annihilate and release about 22 kilotons of TNT-worth of energy, almost exactly equal to the Fat Man atom bomb that destroyed Nagasaki. And the 1 gram of regular matter that the antimatter annihilated would also release 22 kilotons of TNT-worth of energy. –  Andrew Jul 1 '11 at 2:48
    
Even a microgram would be like a backpack bomb full of TNT. –  Florin Andrei Jul 11 '11 at 21:57

We also make use of anti-matter daily in Medical physics in the Positron Emission Tomography (PET)

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Thanks for your input; it would be excellent if you could find the time to elaborate so that the community has your key points at hand from the linked article. –  Grant Thomas Jul 11 '11 at 7:30

If you're looking for a direct experimental demonstration, rather than references to existing research, the simplest way is probably to build a cloud chamber. A basic one can be built with little more than materials from a high school chemistry lab. Potassium chloride salt substitute is an easily obtained positron source.

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Is it from decay of potassium-40? –  Peter Mortensen Aug 4 '11 at 16:22
    
@PeterMortensen: Yes. –  dmckee May 29 '12 at 1:37

When we look towards the center of our Milky Way galaxy (in the constellation Sagittarius), we can detect gamma-ray emission of energy 511 keV (kilo electron volts) which represents the energy of annihilation of an electron and a positron. A positron the anti-matter equivalent of an electron, and has a positive charge instead of a negative charge. Electrons and positrons each have rest mass of 511 keV (from Einstein's mass-energy equivalence E = mc^2) and when they meet, they mutually annihilate, giving off two of these 511 keV gamma rays.

So the fact that we observe these 511 keV gamma rays is proof of the existence of anti-matter in the universe at large, and not just in our accelerators.

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Uh... the electron is 511 keV, not 507. Is the 4 keV difference due to gravitational or doppler redshifting? –  Andrew Jul 29 '11 at 17:45
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Thanks, Andrew; I should have used 511 keV and I have edited my answer; the original observations must have been at 507, but I don't know whether the difference was due to limited spectral resolution of the experiment, or due to a redshift due to gravity or radial velocity. –  Pete Jackson Aug 3 '11 at 21:40

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