Antimatter. It has the same mass as regular matter, and extremely similar energies. Its one significantly different quality is that it has the opposite charge of ordinary matter. When antimatter particles and regular matter particles interact, the result is
a) the complete destruction of all particles involved and
b) the emission of highly dangerous gamma rays when said particles contact each other.
When one gram of it comes into contact with regular matter, the result is an explosion that would release as much radiation and have the same destructive power as two nuclear bombs. However, we do not need to fear that a massive explosion will occur at the CERN supercollider, because we can’t make enough of it to cause a significantly destructive blast. Although we currently cannot make enough in order to produce a nuclear holocaust, antimatter still has interesting uses in medicine and rocket fuel.
There are theoretically equal amounts of matter and antimatter in the universe. However, our Earth and our solar system is somewhat devoid of significant amounts of it. Antimatter is currently being produced at the aforementioned CERN supercollider, a large particle physics laboratory in Switzerland. Scientists at CERN have made a few dozen anti hydrogen particles, and are using some for further research, primarily regarding detection of antimatter particles and reactions in outer space. Antimatter-matter reactions are not only used for observation purposes at CERN. Antimatter particles are used in Positron emission tomography, a medical imaging system. This involves the production and use of positively charged anti-atoms. These are very commonly made for medical use. Also, ion therapy, using anti-particles, is a treatment method for some cancers. In addition, it is theoretically possible to power high-speed extraterrestrial travel using energy from a series of antimatter-matter collisions. However, this is far ahead of our current technology.
Overall, antimatter is the opposite of regular matter. Somewhat small amounts of it, although beyond our current production capability, could cause significant death and destruction. However, it has interesting practical uses. Later generations could potentially use antimatter-matter reactions to fuel jaunts to other, more distant, portions of space. These reactions also are currently helping fight one of the most tragic diseases to plague our species. It seems like antimatter is a double-edged sword: with one side having the potential to destroy the world and the other being able to work medical wonders and give physicians further insight into the human body.