Something else that has not been mention in detail here is anti-particles, which basically are what anti-mater is made of. An example is the electron, which has an anti-particle the positron. Particles have the same mass as their anti-particles but are opposite in charge and other attributes. A positron has a positive charge, the opposite of an electron. If a positron and electron ever touched they would annihilate each other, creating photons. However to create a particle/anti-particle pair you have to make two massless particles, such as photons, collide with a certain amount of energy so that the mass can be created.