Even between clusters of galaxies, there is a thin plasma of ionised hydrogen. It would be expected that there would be gamma emission from the boundaries between the regions dominated by matter and anti-matter but searches have not found any excess.

Thanks for the answer! I have the same guess and now curious how solid the fact you mention about hydrogen. Is it still possible to have large volume around such hypotetical galaxy without any matter at all or our current knowledge dictate against it?

It is hard to observe but here is a basic article and some more advanced references. I haven't studied this in detail, these are just from a cursory Google search.

https://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic_medium

http://iopscience.iop.org/article/10.1086/320548/meta

http://iopscience.iop.org/article/10.1088/0004-637X/769/1/25

https://www.space.com/8386-huge-chunk-universe-missing-matter.html

My theory provides a simple Cosmogenesis that explain why we only see matter in this Universe.

https://hypergeometricaluniverse.quora.com/Is-the-Big-Bang-Illogical

https://hypergeometricaluniverse.quora.com/Is-the-Big-Bang-Illogical