If the planets were exactly symmetrical, nothing would exist (planets, particles, people, etc.) because there would have been an equal # of particles and antiparticles, which together quickly annihilate, as in the production of gamma rays. It would be a universe filled with radiation and no cap't crunch (atoms).

Virtually no antimatter exists in our universe, however, and scientists are stumped to as why that is.

Physicist Alan Guth's inflation theory should have created equal amounts of matter and antimatter

Of course if there were equal amounts of matter and antimatter, I wouldn't be here to ask these questions.

So how did matter survive annihilation?

http://cossc.gsfc.nasa.gov/epo/brochures/new_win/nw4.html

http://www.infn.it/news/newsen.php?id=343