It is not known whether antimatter is repulsed by or attracted to matter. For the purpose of this question, I assume that antimatter is repulsive to regular matter (otherwise this question makes no sense). This debate is not the point of the question.
If there was a massive (much larger than a galaxy) amount of antimatter at the center of the known universe, it would cause expansion of galaxies around it since it's repulsive force would be greater than galaxies attractive forces. This seems like a simpler explanation than positing the existence of dark energy. However, I am not knowledgeable enough to know if the kind of repulsion that would cause at all fits the repulsion that is actually observed.
Could a super massive amount of antimatter (assuming anti-gravity) cause the actual observed expansion of the universe?