That first example was predicted by Paul Dirac as a "hole" state of negative energy.
Observations of interference between various types of kaons led Good to conclude that this stuff does not experience gravitational repulsion. The ATRAP and ATHENA projects are working on methods of containing its atoms, with the goal of using it to test the CPT theorem. Some of it was synthesized following the Big Bang, but no remnants have been observed. The first example of it was discovered by Carl Anderson during observations of cosmic rays with sufficient energies to undergo pair production. That first example was predicted by Paul Dirac as a "hole" state of negative energy. FTP, name this material which undergoes annihilation when it combines with regular matter.
Observations of interference between various types of kaons led Good to conclude that this stuff does not experience gravitational repulsion.
antimatter