Yes, it is strangely unusual if you follow baseball even casually not to have heard of Sammy Sosa. Did you follow ball in the late 90s? His homerun rivlary with Mark McGwire in 1998 was legendary, with McGwire hitting 70 and Sosa finishing with 66. This was before Bonds started peaking. Ken Griffey Jr., and Rafael Palmeiro were (some of) the other big sluggers of that era.
As a sidenote, Sammy Sosa was apparently known enough that in Hungary (where I lived from '98-'03), there was a local punk band named after him (and not a band of ex-pats, but local Hungarians.)