Antimatter
Matter composed of antiparticles — for every regular particle, there is a mirror partner with opposite charge. Annihilates with normal matter on contact.
Antimatter is composed of antiparticles, each the mirror counterpart of an ordinary matter particle with the same mass but opposite electric charge. The antiparticle of an electron is a positron; of a proton, an antiproton.
When matter and antimatter meet, they annihilate, converting their entire mass into energy according to E=mc². One gram of antimatter meeting one gram of matter would release roughly the energy of 43 kilotons of TNT.
Antimatter was predicted by Paul Dirac in 1928 and discovered (the positron) by Carl Anderson in 1932. It's used today in PET (positron emission tomography) medical scans. The cosmological mystery of why our observable universe is overwhelmingly matter is one of physics' biggest open questions.