The word is a derivative of Proto-Indo-European word *ma- . Therefore, many languages that are part of Indo-European family use the same word, sometimes slightly modified.
There is a theory, the word MAMA comes from the period of babbling ( a stage in child development and a state in language acquisition during which an infant appears to be experimenting with uttering articulate sounds). During that period they are experimenting with their voice. Thus the word "Mama" is a result of those experiments.
Well, but Swahili is not Indo-European, but a Bantu language with arabic influence...