Matter and antimatter
For every type of matter particle we've found, there also exists a corresponding antimatter particle, or antiparticle.
Antiparticles look and behave just like their corresponding matter particles, except they have opposite charges. For instance, a proton is electrically positive whereas an antiproton is electrically negative. Gravity affects matter and antimatter the same way because gravity is not a charged property and a matter particle has the same mass as its antiparticle.
When a matter particle and antimatter particle meet, they annihilate into pure energy!