What is an alternate Reality? Such worlds exist on two levels, the physical and the metaphysical.
Physically, each alternate Reality is a collection of planes or dimensions. Our universe, out to the limits of space, is one plane. It contains countless stars and many, many galaxies. If there are other planes, we don’t know of them. That’s our Reality.
An alternate Reality might have a primary plane resembling that, plus a Heaven and Hell (resembling medieval conceptions of such, ala Bosch). Or, its primary plane might be a sphere, in the center of which is a flat Earth surrounded by crystal spheres, on which planets are located, all of them (plus the sun) orbiting around the Earth on epicycles. That’s an alternate Reality: an entire separate cosmos, full of impossibility.
When it comes to alternate Realities, there are no universal constants. Even the laws of physics which hold in our universe, need not necessarily be true. Magical forces, Divine forces, even moral imperatives can be as fundamental a part of the alternate Reality as the Laws of Thermodynamics are a part of our Reality. In another Reality, Good and Evil might not be moral precepts, but forces as real as nuclear fusion, as effective, and as powerful. (These facts are not well-understood by Infinity, Inc., and though some have theorized that this is the case, these theories are not accepted outside a small community of two or three cranks.)
In any given Reality, it may be possible to travel between planes. Angels may carry souls to Heaven, warlocks may summon creatures from outside time and space, and starships may easily enter and exit hyperspace. The walls between Realities are inviolate, however.
No known spell, miracle, invention or artifact can penetrate the walls between Realities. Only the (seemingly) natural portal events can establish contact between one Reality and another.
What causes them is unknown. Why they are so variable is unknown. And why they are increasing in frequency is (most worryingly) unknown.