It is quite complicated. At the big bang, the universe can be considered to start out as pure energy. At sufficiently high energy (at the big bang, it was more than high enough), photons can collide and produce quark-antiquark pairs or lepton-antilepton pairs. However, there is a very slight difference in the decay patterns of matter versus antimatter. As a result, as things cooled down, there was a slight net remnant of matter.
Astrophysicists are still struggling to work out the exact details, but there is general agreement on the idea described above.

• Yes, correct - alexander - April 10, 2001 - 00:33 UTC
  • Matter-Antimatter Fuel Big Bang?? - admin - April 13, 2001 - 23:31 UTC
    • energy-stuff equilibrium - mathman - April 14, 2001 - 04:02 UTC
    • Or fuel Inflation? - bladesinger - April 23, 2001 - 13:31 UTC
      • Higgs Field and CPT violations - mournblade - May 4, 2001 - 07:48 UTC
    • Matter-Antimatter Fuel Big Bang - supernova - May 14, 2001 - 22:52 UTC
      • Matter turns in radiation back and forth. - alexander - May 15, 2001 - 01:43 UTC
  • Galaxies contain similar quanitities of matter and antimatter - normhansen - July 6, 2001 - 21:48 UTC
    • carzy idea - mathman - July 7, 2001 - 04:02 UTC
      • Well actually... - seyfert - September 10, 2003 - 10:38 UTC
    • Stars forms from H left over after Big Bang. And antistars? - alexander - July 20, 2001 - 17:52 UTC
      • not so sure - seyfert - September 10, 2003 - 10:47 UTC
    • Galaxies contain similar quanitities of matter and antimatter? - seyfert - September 10, 2003 - 10:34 UTC
  • left overs ? - perseus - August 5, 2001 - 11:05 UTC