Antimatter is a material composed of antiparticles, particles that share almost all physical features of a substance. However, it has a reverse charge referent to its corresponding antiparticle. Also, when matter touches antimatter, both are annihilated.
Antimatter is a rare substance in our universe (which can be artificially created) used in power generation for ships, cities, and even entire planets when stars are too far away for Dyson Spheres to be effective. The power generation comes from matter-antimatter reactions, that create a great quantity of energy.
When normal matter and antimatter come into contact with each other, they annihilate in a burst of energy. This energy can be utilized to do many things, but mostly for power generation in ships and the like.
Naturally occurring Antimatter
Few places in the universe are composed entirely of antimatter leftover from the Big Bang, the exception being the Pak Supercluster, a region of about 300 galaxies 300 gigalightyears from the Milky Way. Elsewhere throughout the universe, many antiparticles are constantly being made and destroyed. For example, when high energy cosmic rays hit the atmosphere of the Earth, some quantum reactions take place resulting in an undetectably small amount of antimatter to be produced, which annihilates instantly with normal particles. Quasars emitted from some black holes also can produce minute amounts of antimatter.
The matter "monopoly" in the universe has a simple reason: randomness. Subatomic particles can move randomly, thanks to the uncertainty principle. These random movements simply cooperated for a cascade effect that increased the percentage of matter in the universe exponentially.
History of Use
The term antimatter was first coined in 1898 by Arthur Schuster in two interesting letters to a magazine called nature. The letters discuss the possibility of "antiatoms" and entire solar systems made of antimatter. This was not a serious scientific proposal, but rather a speculation. Schuster also speculated about antimatter having negative gravity, which was later disproven.
The modern theory of antimatter began when a paper was published by Paul Dirac discussing the concept. The paper predicted the existence of antielectrons being produced in a quantum reaction, which was later proven to be so by Carl D. Anderson in 1932. 
In 1995, CERN announced that they created nine antihydrogen atoms, making them the first humans to create anti-atoms. Later, in 2002 project ATHENA created antimatter which was "cold" enough to study effectively. 14 years later in 2016, humanity discovered a way to more effectively create antimatter than before via the ELENA system.
More efficient antimatter production
In 2032, the Japanese team simply called Nihon no Tanmonoshitsu (Japanese Antimatter) figured out a way to produce antimatter at a cost of 3 Billion dollars per kilogram, which was a huge improvement over the previous estimate of quadrillions of dollars per kilogram. This antimatter was used in the first probe to reach Proxima Centauri, called the Daedalus Missions.
In 2111, the scientist Itsuki Fujibayashi had the idea to compact particle accelerators into a very small space, which would drastically lower the price of antimatter production. After ten years, the first accelerators where online and the company Fujibayashi Antimatter was born. The company quickly grew, absorbing many smaller companies.
Antimatter power plants
In 2158, the first power stations utilizing antimatter came online on Triton, as solar power was not efficient that far away from Sol. This was a huge milestone for Fujibayashi Antimatter, as before then the demand for antimatter was relatively low, as only long-distance probes and generation ships needed it in meaningful quantities.
The pure warp drive does not require any energy, as it is a reactionless drive. However, it is too slow, so, to increase the speed, ludicrous amounts of energy are required. So much so that solar panels could never hope to provide the required amount. With the invention of the warp drive, antimatter, the only fuel dense enough to be practical, became one of the best selling substances ever. To this day, antimatter is widely used as a fuel source, powering virtually every ship in existence.
Antimatter and normal matter are both affected by gravity in the same way. They also are affected by all other fundamental forces in the same way. A star made entirely of antimatter would undergo nuclear fusion just like a normal matter star.
Antimatter is not absorbed by strange matter. Instead, they simply annihilate each other. Strange antimatter can absorb antimatter, although this substance is difficult to produce cost-effectively.