The many dimensions that are more subtle than the physical world, which is why they are called "internal," since they are "within" physicality. All dimensions are here and now, interpenetrating.
There are subjective (projected, illusory worlds) and objective worlds (real, authentic). To know the objective internal worlds (such as the Astral Plane, or Nirvana, or the Klipoth) one must first know and discriminate one's own personal, subjective internal worlds, because the two are intimately associated.
"Whosoever truly wants to know the internal worlds of the planet Earth or of the solar system or of the galaxy in which we live, must previously know his intimate world, his individual, internal life, his own internal worlds. Man, know thyself, and thou wilt know the universe and its gods. The more we explore this internal world called “myself,” the more we will comprehend that we simultaneously live in two worlds, in two realities, in two confines: the external and the internal. In the same way that it is indispensable for one to learn how to walk in the external world so as not to fall down into a precipice, or not get lost in the streets of the city, or to select one’s friends, or not associate with the perverse ones, or not eat poison, etc.; likewise, through the psychological work upon oneself we learn how to walk in the internal world, which is explorable only through self-observation." —Samael Aun Weor, Treatise of Revolutionary Psychology
Through self-observation, we develop the capacity to awaken where previously we were asleep: including in the objective internal worlds.
The internal worlds are mapped on the Tree of Life of Kabbalah.