A common hypothesis among people who know about these things, is that In The Beginning there was an anti-particle for every particle. These anti-particles and their matching particles were annihilated in the Big Bang, and the material universe we live in today is the leftover particles, a surplus so to say. Some small antimatter particles – positrons – buzz around all the time, and by much effort scientists have created anti-protons in the laboratory. Antimatter is basically the same particle with the opposite charge – a negative proton, a positive electron.
However, some people think that there might be pockets of antimatter left in the universe. This leads to a whole lot of speculation upon various subjects, including the possibility of using antimatter to fuel starships, but also if there might be things like anti-stars out there – a star made out of oppositely charged (to what we are used to) particles for instance.
Maybe there is an anti-Earth out there. Maybe an anti-Silme. I sincerely hope she is busy studying superconductivity and other important issues, because as the years pass I have to admit that the likelyhood of this version of Silme becoming an astronomer and physicist is dropping for every year.
But a girl can dream, eh?