Gaia, or Anotherworld, is the source of all magic and the parallel universe responsible for the presence of magic on Earth.
On Gaia, the vast majority of humans were killed at a distant point in the past. It exists in a permanent technological dark age. The planet is colder, the oceans are lower, and the natural world has grown unimpeded for many centuries. Gaia is a world brimming with magical energy— and without any magic-wielders to use up its ever-growing source of magic, the planet has been flooded in it. After a certain point, the magic became so concentrated that it began spilling out into the space between realities. In other words, their world began to overlap with ours.
With strong enough control over magic, or the guiding hand of one of Gaia's creatures, it's possible to travel to Gaia from our world— and from there, to anywhere else in the connected universe.
In 2006 (our time), Gaia's magical energy reached its limit. The fabric of its reality could not contain it, and so the magic spilled out of that reality and found its way into others. A link was formed between them and Gaia, a steady flow of pure magic that saturated each new world that it connected to.
On our world, this caused humanity to awaken to its magical potential. Small communities of these magical humans came together, found each other, and started working together. Magic has been a poorly-kept secret ever since.