What's the term for not specifying race or gender when it is the majority?

This may be specific to the jargon of Critical Theory as inherited by SJWs, but in those contexts I’ve seen the term normative used. For (a contrived) example, a work marking left-handedness but leaving right-handedness as the unmarked default might be said to be “dexter-normative.”

Marked/unmarked is indeed a well-established terminology for this, in many fields — originally linguistics, and from there, various social sciences. From Wikipedia, Markedness:

In linguistics and social sciences, markedness is the state of standing out as unusual or divergent in comparison to a more common or regular form. In a marked–unmarked relation, one term of an opposition is the broader, dominant one.

In particular, in the section on Cultural markedness and informedness:

Linda Waugh extended this to oppositions like male/female, white/black, sighted/blind, hearing/deaf, heterosexual/homosexual, right/left, fertility/barrenness, clothed/nude, and spoken language/written language. Battistella expanded this with the demonstration of how cultures align markedness values to create cohesive symbol systems, illustrating with examples based on Rodney Needham's work.

The Wikipedia article isn’t very comprehensive, and doesn’t say much more about the specific kind of usage you describe — but I’ve heard this terminology very widely used in that sense, in both technical and non-technical contexts.