I have learned about the existence of positrons as a decay product from uranium fission - if I'm not mistaken. Is there any evidence for higher 'mass' anti-matter, or is that mere speculation or rationalisation? i.e. Is the argument that there anti-matter based planets in the universe, or just a soup of positrons?
Your question suggests that you think positrons are the only form of anti-matter, but as far as we know every particle has a corresponding anti-particle (some particles like the photon or Z are their own anti-particle). The positron weighs about 0.5MeV while the heaviest known anti-particle is the anti-top quark at 173 GeV i.e. 346,000 times as heavy as the electron.
Anti-hydrogen atoms have been made. These contain a positron bound to an anti-proton, so these are the simplest form of anti-atoms. The anti-hydrogen atoms are too hard to handle to assemble into anti-molecules so we haven't seen any form of anti-matter more complicated than a hydrogen atom yet.
The final part of your question, whether there are anti-matter planets, is addressed in the link Qmechanic posted in his comment. The headline is that there is no evidence for any sizable (i.e. bigger than a hydrogen atom) aggregation of anti-matter anywhere in the universe.