I mean in the amount of energy it has and in its size. That which is observable is apparently only a sub-universe. It seems to be limited only by physical laws, but even those laws could be different in other sub-universes.
I don't know about any differences. Particle physicists don't have much of an opinion on gravity and on uniting gravity with the other forces.
A proton and neutron can still have three quarks each with some of them being matter quarks and others being antimatter quarks. The strong and weak forces can be manifestations of gravity and electromagnetism at short scales, and electromagnetism could be a manifestation of gravity and anti-gravity, but particle physicists generally do not concern themselves with such ideas.
There's a lot of resistance to the idea of antimatter having antigravity.
The antimatter gravity debate
When antimatter was first discovered in 1932, physicists wondered about how it would react to gravity. Initial analysis focused on whether antimatter should react the same as matter or react oppositely. Several theoretical arguments arose which convinced physicists that antimatter would react exactly the same as normal matter. They inferred that a gravitational repulsion between matter and antimatter was implausible as it would violate CPT invariance, conservation of energy, result in vacuum instability, and result in CP violation. It was also theorized that it would be inconsistent with the results of the Eötvös test of the weak equivalence principle. Many of these early theoretical objections were later overturned.[3]
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Morrison's Argument In 1958, Philip Morrison argued that antigravity would violate conservation of energy. If matter and antimatter responded oppositely to a gravitational field, then it would take no energy to change the height of a particle-antiparticle pair. However, when moving through a gravitational potential, the frequency and energy of light is shifted. Morrison argued that energy would be created by producing matter and antimatter at one height and then annihilating it higher up, since the photons used in production would have less energy then the photons yielded from annihilation.[4] However, it was later found that antigravity would still not violate the second law of thermodynamics.[5]
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The equivalence principle If one can invent a theory in which matter and antimatter repel one another, what does it predict for things which are neither matter nor antimatter? Photons are their own antiparticles, and in all respects behave exactly symmetrically with respect to matter and antimatter particles. In a large number of laboratory and astronomical tests, (gravitational redshift and gravitational lensing, for example) photons are observed to be attracted to matter, exactly in accordance with the theory of General Relativity. It is possible to find atoms and nuclei whose elementary particle contents are the same, but whose masses are different. For example, Helium-4 weighs less than 2 atoms of deuterium due to binding energy differences. The gravitational force constant is observed to be the same, up to the limits of experimental precision, for all such different materials, suggesting that "binding energy"— which, like the photon, has no distinction between matter and antimatter— experiences the same gravitational forces as matter. This is again in accordance with the theory of General Relativity, and difficult to reconcile with any theory predicting that matter and antimatter repel.
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Schiff's argument Later in 1958, L. Schiff used quantum field theory to argue that antigravity would be inconsistent with the results of the Eötvös experiment.[6] However, the renormalization technique used in Schiff's analysis is heavily criticized, and his work is seen as inconclusive.[3]
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Good's argument In 1961, Myron Good argued that antigravity would result in the observation of an unacceptably high amount of CP violation in the anomalous regeneration of Kaons.[7] At the time, CP violation had not yet been observed. However, Good's argument is criticized for being expressed in terms of absolute potentials. By rephrasing the argument in terms of relative potentials, Gabriel Chardin found that it resulted in an amount of Kaon regeneration which agrees with observation.[8] He argues that antigravity is in fact a potential explanation for CP violation.