The Kah'mihrkal, or sometimes called "The Higher Plain", is a dimension, much similar to fluidic space.
What is known is that the Orgyi and the Rifteons come from this area, and perhaps even the humanoid race believed to have seeded the galaxy.
According to the general belief of the Federation, mankind has achieved a state of demi-godhood, where they are united and have shrugged off all forms of vice and violence. Regardless, there are some who believe that total godhood is a state that mankind SHOULD strive for. The proto-humanoids who left a message found by Jean-Luc Picard in the mid-24th Century had guessed that all races would want this, and so they left forged messages that declared themselves as the progenitors of all the races of the galaxy. This message later led to another one that later would open a door-way into the "Higher Plain" for the races to achieve a state of godhood.
However, no humanoid has ever returned from the Higher Plain, so it is not known whether or not this is truth or lies. But, in one incident, the leaders of many of the races of the galaxy were halted from making a joint-crossover into the Higher Plain because it was feared to be a trap.