Center of galaxies

Whenever there's a gravitationally "stable" system with objects
revolving around a center of mass and there's no wobble on the object
which sits close to the center of mass, this must mean (according to my
meager physics years ago) that this object must posses mass at least
equal to the sum of masses of all the revolving objects. (or that such
an object must have mass much larger roughly speaking than the mass of
the rest of the members).

So, speaking of a galaxy containing 2 billion solar masses in official
nomenclature, doesn't it follow that "something" sits at the center of
mass of that galaxy which contains _at least_ 2 billion solar masses in
order to sustain the system gravitationally?

I've heard of theories that claim that black holes lie in the center of
many galaxies, but it seems to me that one probably sits at the center
of _every_ galaxy. Correct me if I am wrong, but this is mind boggling.
I just saw some diagrams of some giant doubled lobed galaxies, like DA
240 and 3C 236 (Cygnus A?). Whatever the heck sits at the center of mass
of those giants?
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
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Eventually, _everything_ is understandable