Portal

Portals in Metroid Prime 2: Echoes transported Samus from Aether to Dark Aether, or vice-versa.

There are two kinds of portals: full, man-made portals, and naturally occurring dimensional rifts.

Portals
Portals were built by the Luminoth during the war. The first portal discovered by Samus, in the Agon Wastes, had to be activated through a series of tasks in order to get it solar energy. These portals are always two-way (entering a portal on Aether brings Samus to a portal on Dark Aether, so she can return immediately) and the ones on Dark Aether are invariably protected by a field of light. They must be activated by being scanned.

Rifts
Rifts are naturally occurring portals, appearing as flattened, swirling vortexes. They are activated by firing energy into them. Light rifts, to access Aether, are found on Dark Aether and are activated by firing the Light Beam at them, and Dark rifts, to access Dark Aether, are found on Aether and are activated by shooting them with the Dark Beam.

Rifts can be one-way, that is, entering a rift on Aether does not necessarily lead to one on Dark Aether, so Samus may have to discover one on her own, and can't return home immediately.