A hydrogen atom typically consists of one proton and one electron.
The opposite to a proton is an antiproton and the opposite to an electron is a positron.
They DO exist. They are just incredibly rare as the overiding abundance typically belongs to normal protons and electrons and such.
When an anti proton meets it's opposite constituent, (the proton), they 'Annihilate'
This is the general trend between all fundamental particles.
There IS such thing as an anti-hydrogen atom.
It is composed of a positron and an antiproton.
Upon meeting it's opposite constituent, (the Hydrogen atom), they will annihilate.
There are theories that entombing every galaxy is an invisible wall of dark matter which cancels out the electromagnetic effects of all fundamental particles.
Therefore it is very likely that somewhere out there is a galaxy which is entirely made up of antimatter.
What happens if our galaxy collides with that one?
Theoretically; an explosion which could rip the fabric of space-time so severely, the universe will collapse in on itself.
Try that for a slice of fried gold; I win the awesome facts game.