We're pretty darned sure this can't be the case. As mathman noted, if this were the case we would see a lot of matter/anti-matter annihilations. We have looked for these, by the way, and don't see them.
But perhaps even more compelling is the existence and smoothness of the cosmic microwave background: this stuff was emitted when our universe cooled from a plasma. At the time, our universe was uniform to one part in 100,000. It was almost perfectly smooth. Because of this, there just wasn't anywhere for the anti-matter to "hide" from the matter to avoid annihilation. So there really can't be much of any anti-matter left around.