Matter and Antimatter
Whenever energy is converted to mass, for every matter particle created, an antimatter particle is created as well. Thus, every particle has an anti-matter equivalent. If a particle and its antiparticle were to collide, they would both be annihilated, and disappear in a flash on energy. If you think back to the cookie cutter analogy earlier, annihilation is like putting the cookie back into the anti-cookie to form the original sheet of energy.
Currently, the most accepted theory on the creation of the Universe is the Big Bang. We can observe that all galaxies are moving away from each other, so they must have all shared the same position about 10 to 15 billion years ago. Scientists have concluded that all of the Universe was once in a single point, and expanded in a violent explosion they have named the Big Bang. Ten billionths of a second after the Big Bang, the Universe would have been small enough to fit in your room. Inside it, particles were travelling at tremendous speeds, allowing them to easily create new matter and anti-matter. Numerous particles and anti-particles were constantly being created - and constantly being annihilated back into energy.
For a still unknown reason, a small surplus of matter was being created. For every one billion anti-matter particles created, one billion and one matter particles formed. Within a second, the anti-matter was annihilated by the matter. The Universe had then expanded so much that the kinetic energy of the particles was too low to create new matter and anti-matter. Only the extra matter remained, along with the radiation energy created from the annihilation of the anti-matter. We can still detect the echo of the annihilation of anti-matter by matter by detecting the energy created. The satellite COBE (COsmic Background Explorer) has imaged the radiation coming from all directions of the Universe:
Continue to the history of how we discovered antimatter.