Why are matter and antimatter not coming into contact within our atmosphere or within close proximity to earth. Obviously I understand the meeting would be catastrophic but why does it only happen in distant galaxies. Please excuse my ignorance.
Antimatter is very rare in our universe as it mostly contains normal matter. It is also not true that only distant galaxies produce antimatter. There is antimatter in our galaxy too. The fact is that there is so much more matter that the distribution of antimatter is vastly spread out and although they may be produced, antimatter does not stay antimatter for significant times as there is so much matter around( they collide with matter again to form gamma rays). Thus altogether they don't pose a threat as there is so little of it around that finally reaches earth.
This link below is an article about antimatter detection in our own galaxy. Together in cosmic rays we also get antimatter detection but it makes up less than 1% of cosmic rays and cosmic rays are the collection of the stream of particles from the entire universe. So thus you can grasp the situation here.
Despite being relatively rare compared to matter, antimatter is all around us, as a result of cosmic ray particle showers. A very small proportion of cosmic rays are positrons, but mainly they are baryons. They hit the atmosphere and form cascades of particle-antiparticle pairs, the antiparticles annihilate with other particles making more pairs, and so on. Antiparticles are hitting your body almost every second.
Because of baryogenesis, the production of matter over antimatter early in the universe, we don't expect distant galaxies to be different. Although given the assymetry is not fully explained one possibile reason for could be https://en.m.wikipedia.org/wiki/Baryon_asymmetry#Regions_of_the_universe_where_antimatter_dominates