Antimatter is the collection of particles that correspond to the particles of matter but that have opposite electrical charges of their normal matter paired particles. Antimatter and matter annihilate each other when they meet because opposite charges neutralize each other. It is theorized that at the origin of the universe, matter was favored somehow (most likely through the Higgs boson) so that there was more matter than antimatter, otherwise equal quantities of matter and antimatter would have self-annihilated leaving a universe of electromagnetic radiation without matter.
[T]here would be essentially no matter left around; annihilations would have converted everything into electromagnetic radiation by now. So clearly this imbalance is a key property of the world we know. (Barnett and Quinn, "What is antimatter?")
Antimatter is antagonistic to normal matter, hence, the properties of subatomic particles of normal matter are opposed to the properties of particles of antimatter. The electrical charge of particles of antimatter is turned around with respect to matter. As examples, positrons, or anti-electrons, are positively charged while anti-protons are negatively charged. For every matter particle there is a corresponding, oppositely charged, antiparticle.
The existence of antimatter partners for all matter particles is now a well-verified phenomenon, with both partners for hundreds of such pairings observed. (Barnett and Quinn)
Antimatter is also subjected to gravity force, as matter is. The antimatter particles are the results of high speed collisions and first, after the Big Bang, it is theorized that equal quantities of antimatter and matter particles were produced. The question that scientists do ask is why matter became dominant, while antimatter became rare. Current particle collisions attempt to prove that the Higgs boson provides the missing reason for why matter dominated over antimatter (it has recently been found that antimatter particles have a slightly lesser mass than their corresponding paired matter particles). The collisions between antimatter and matter particles produce energy.
The existence of antimatter was first theorized in 1928 by English physicist Paul Dirac, while, in 1931, American physicist Carl D. Anderson first discovered an anti-electron called a positron. Electrons of matter are negatively charged but anti-electron positrons are positively charged. All antimatter particles similarly carry opposite charges to their paired particle even though their other properties nearly perfectly correspond to matter particle properties.
[T]he laws of physics for antiparticles are almost identical to those for particles; any difference is a tiny effect. (Barnett and Quinn)