Im no expert but I think I know current theory, if anyone knows I'm wrong please point it out:
Our universe, or at least the universe we percieve is a universe of matter. Doing thing fantastically clever that I will never understand, scientists have worked out that our univers's mass is 10% of what it should be. i.e. we can't see 90% of the matter of our universe, hence dark matter, and that's about as much as I know about dark matter.
Now we know due to stellar evidence at the time of the great ionisation (when the suns were first born they let out a radiation so intense it stripped molecules of their electrons, turning the universe into a soup of super charged ionised molecules) that molecules that have charge have a mass, and for every molecule of a certain charge and mass, its opposite exists. i.e. protons and antiprotons, electrons and postirons, helium and antihelium, hydrogen and antihydrogen. These examples of antimatter have all been recreated in laboratories (albeit in MICRO amounts) proving their existance.
So now we know there's a universe of antimatter somewhere. Scientists are again currently doing something very fantastic and complicated to try and detect this antimatter universe, to see if one even exists (something to do with detecting that special radiation i mentioned before).
So there are apparently 2 universes, each are still expanding due to the energy of the big bang, and as Marcus quite rightly put it, there will be a "Big Squeeze" to compliment the "Big Bang". Hopefully this won't be any time soon, by the time it does happen we will have bled ourselves dry and the human race will be dead and gone and faded to dust. Thoguht I'd end on a nice cheery apocalyptic note
