Do experts in the field know the gravitational effects of large amounts of antimatter.

Would antimatter be self repulsive? Woud antimatter attract antimatter and be repulsive to ordinary matter? Would antimatter attract antimatter & ordianry matter?

AFAIK antimatter reacts normally to gravity, it just has opposite charge on atomic/ sub-atomic components.
It still has mass rather than "anti-mass".

Oli si correct. Gravity makes no distinction between matter and anti-matter.

I believe the previous posts.It still has mass rather than "anti-mass".

Oli si correct. Gravity makes no distinction between matter and anti-matter.I wonder how the above is known.

I cannot imagine any experiement sensitive enough to measure the gravitational effects of the tiny amounts of antimatter created by particle accelerators.

Therefore there must be some theory based reason for the belief. What is it?

Because antimatter varies from ordinary only in the charges of its component particles for one.

The only direct experimental result on antimatter and gravity comes from Supernova 1987A. This supernova in the Large Magellanic Cloud emitted both neutrinos and antineutrinos, some of which were eventually detected on Earth. Those neutrinos and antineutrinos took 160,000 years to reach Earth, and while travelling were bent from a "straight line" path by the gravity from our own galaxy. The bending with gravity changed the time needed to reach Earth by about 5 months, yet both the neutrinos and the antineutrinos reached Earth at roughly the same time (within the same 12 second interval). This shows that the neutrinos and antineutrinos "fell" similarly, to a very high level of precision (about 1 part in a million). [4] and [5] provide some background information on this.
from http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/antimatter_fall.html
but yes, it is something more assumed at the moment rather than experimentally verified.

Oli: Thanx for the info, indicating some experimental evidence to back up theory.

It is interesting that the antineutrinos made it so far without encountering ordinary matter & being annihilated.

I wonder how the above is known.

I cannot imagine any experiement sensitive enough to measure the gravitational effects of the tiny amounts of antimatter created by particle accelerators.

Therefore there must be some theory based reason for the belief. What is it?

Well, think about it. What does gravity couple to? Mass. In some sense, gravity doesn't KNOW about electrical charges---gravity ONLY cares about a particles mass. And if I tell you (or you can go preform an experiment) that the mass of a particle and an anti-particle are the same, what does that say about the gravitational interactions?

BenTheMan: I like your thoughts and know of no reason to believe that antimatter interacts repulsively with itself or ordinary matter. Still some experimental evidence always makes me more confident that the best theories & thought experiments.

Still some experimental evidence always makes me more confident that the best theories & thought experiments.

I don't think there are any experiments which would prove this. The best I can say is that general relativity tells us that gravity couples to the stress energy tensor, which only cares about energy density, and doesn't know anything about electrons or positrons---both have positive (definite) energy density, so both couple to gravity in the same way.

In this sense, you're violating GR if you couple matter and anti-matter to gravity differently.

BenTheMan: Did you notice the post by Oli? That seems to be experimental evidence.