For each matter particle, it is predicted that there should be an antimatter particle of the same mass, opposite charge, and opposite other properties. These particles can be created artificially, but they have to be isolated from matter in a strong magnetic field. You are quite right - matter and antimatter cannot exist together in the same place, because they mutually annihilate and create radiation of equal energy to their masses. This was predicted by Einstein in his famous equation, E = Mc² .

Examples of antimatter in laboratories on Earth are at Fermilab, antiprotons are created artificially, and then collided with protons at high energies in the beam collider. When they collide, they annihilate each other in a burst of energy, but other particles are created in the collision, and the properties of those particles are measured with big detectors. You can go to their website to find out more about the process at the Fermilab website here

We believe that in the very early Universe, before the age of the universe was about 1 second old, particle-antiparticle pairs were created out of the radiation field, and then as soon as they were created, they mutually annihilated into radiation again. When the universe expanded and cooled to below the temperature where particle-antiparticle pair production could happen, all the antimatter and matter that were in equal proportions annihilated with each other, leaving only radiation. A little bit of matter was left over - about one particle of matter for every billion photons (that's basically a particle of radiation), which is why we are here!

So, although every matter particle has a theoretical antimatter particle, we only see antimatter rarely in nuclear reactions and particle accelerators, and it seems that antimatter just does not exist for the most part anymore -at least not nearby, or we'd all annihilate into pure energy!

You've probably learned that, when matter and antimatter collide, they turn completely into energy and you're wondering why everything hasn't turned into energy if there is as much matter as antimatter. Is that right? Well, it's a really good question. Particle physicists are still trying to understand the answer.

You see, right after the big bang, 13 billion years ago, there was a lot of energy in the universe and it all got turned into matter and antimatter. Since experiments seem to show that whenever you create matter, you create an equal amount of matter and antimatter, it seems as if there should have been as much matter as antimatter in the universe.

This is a really good question. Physicists have pondered over your question because the laws of nature seemed to obey fundamental symmetries with respect to charge, time, and space. A consequence of these symmetries was that every time a piece of matter was created, a corresponding piece of antimatter was created. Every electron should come with a positron; every proton, with an anti-proton; and so on. So shouldn't they all cancel out in the end?

Good question- if there was one particle of antimatter for each particle of matter, and both were mixed equally in the universe, then everything would annihilate very quickly.

Some scientists used to believe that matter and antimatter became separated somehow, so that there must be faraway galaxies made of antimatter. However, if this were true, hydrogen and anti-hydrogen would inevitably touch and annihilate near the boundary, and we would see the resulting gamma rays. We have not seen the kinds of space gamma rays that come from frequent matter-antimatter annihilation. However, in particle accelerators we have seen an interesting process called CP violation which slightly favors matter over antimatter - in fact, studying CP violation is one of the common uses of particle accelerators. We now believe that when the universe was less than a millionth of a second old CP violation slightly favored the production of matter instead of antimatter. Each type of matter particle still had an associated type of antimatter particle, but they were not produced in equal numbers. There used to be a ten billion times more matter than there is today, but there was only 99.99999999% as much antimatter as there was matter.

Before the universe was a minute old, there was only the slight excess of matter left over- which was enough to make us and everything we see today. If there had not been a particle-physics mechanism favoring one over the other, we would not exist. If it had favored antimatter, we would have simply called antimatter matter, and vice versa- it wouldn't have really changed anything. For more about the early universe and a more thorough answer to your question, read "The First Three Minutes" by Steven Weinberg.

Just for fun I calculated how explosive the matter-antimatter reaction is:

An anti-bacterium could provide enough energy to keep you alive for a day.
A speck of anti-dust could demolish a city block.
4 drops of anti-water would have the power of the Hiroshima nuclear bomb.
A briefcase full of antimatter would turn the LA basin into a sea-filled crater.
A ton of antimatter could provide all the world's electricity and power all gas-burning machines for a year.
17 million tons of antimatter could power the sun for one second.

This is a very good question! It turns out that we believe, in the very early universe, there was a little more matter than antimatter. At at point some time not long after the big bang (within a couple minutes) most of the antimatter and matter combined to make photons, leaving just a little bit of matter left. No one knows yet why there was a little more of what we call matter than anti-matter. This is a hot topic of current research. This is not to say that there is no anti-matter in the universe at all, there is a lot of it being created and destroyed all the time. It's just that the same amount of matter is formed or destroyed at the same time. If, suddenly, all the anti-matter in the universe combined with matter and disappeared, we believe that there would still be some matter left (e.g. galaxies, planets, us).

Antimatter also has mass and takes up space. However, when they meet, matter and antimatter tend to destroy one another with a tremendous release of energy. Why there is so much matter and so little antimatter in the universe is a significant unanswered question in physics. The current theory, as far as I understand it, suggests that a small imbalance toward the production of matter, rather than antimatter, in the early universe may be responsible for the overwhelming prevalence of matter today. Presumably, even a small amount of additional matter back then would result in much more matter than antimatter today. Can you guess why?

Although each particle of matter seems to have an anti-particle, there does not seem to be the same number of both kinds of particles. So the numbers of particles of 'normal' matter seem to outnumber the number of particles of anti-matter by a substantial margin, although the evidence for this is a bit weak for matter are very large distances from us. Fundamentally, although you might expect that matter and anti-matter have opposing properties, this has also been shown to be false. For example, both protons and anti-protons seem to be attracted by gravity.