M Continuum

The M Continuum is the extradimensional plane of existence that is home to the omnipotent race known as "the M." The term is also used to refer collectively to the M themselves. They are considered the "galactic" balance to the M continuum and supposedly appeared in the universe after the manifestation of the Q.

No one except the Q and the M have seen the M continuum but it is presumed to have most, if not all the same properties as the Q continuum. It is "next door" to the Q continuum.

History
According to Q, the M Continuum appeared in existance after the Q continuum at the beginning of the multiverse. The M announced themselves as the enemies of the Q and confessed that they were part of the universe's attempt to provide balance to the Q realm. Their initial reason for hating the Q was "Because there's something about you that really pisses us off", but neither side could see that as an acceptable justification for agressiveness on either side.

After some substantial time pondering, both the Q and the M attempted to enrage the other by coming up with more complicated reasons for hating the other side. The M posited that the Q were the 2nd to emerge into existance and that they were merely copies of the M continuum, so they proposed to destroy the Q. Neither side saw this as an acceptable reason to go to war until one of the M stated as an afterthought, "Your Mother", hostilities then broke out.

However, Q confessed that no one understood exactly why the insult was enough to go to war with the M continuum as no Q knew whose mother was insulted and that no one in the Q continuum had a mother to insult in the first place. the Q and M were simply enemies because "they were".

(TNG novel: I, Q)

The Q Continuum was preceded in terms of, "first of the two to exist," by the Q's neighbors, the M Continuum. (TNG novels: I, Q and Q-Zone)