The Dark Sun, also known as the Dark Star or Ra's Dark Twin, is a zero-point energy field that is the complete opposite to our Sun.
Being that it is the polar opposite to the Sun, it is likely that the Dark Sun came into existence at the same time.
At some unknown point in Earth's history, a race of super-ancient beings discovered the Dark Sun, and through their research learned that it comes closer to the central Sun every few million years, but its deadly energy was blocked from hitting Earth by Jupiter and Saturn. However they predicted that on rare occassions the gaseous planets would not protect Earth, and that any life on the planet at that stage would be wiped out.
They subsequently set about building a Machine to prevent the Dark Sun from doing so, and though they were able to prevent the Dark Star from approaching in their time, the Super-Ancient Beings realised that even after they were gone from the Earth (by extinction, or otherwise departing the planet), there may yet be intelligent life on Earth.
To this end, they made the prevention of the Dark Star the second of five trials which would allow them to determine if a new sentient species did arise and were worthy of existing.
After the Super-Ancients were gone, the Dark Star continued to make the odd breach into the solar system, though the inner planets were largely unaffected by virtue of the gas planets' orbits. However, on one occasion, the Dark Sun's energy destroyed a small planet between Mars and Jupiter, causing an asteroid belt to form.
The Dark Sun is a zero-point fields, and unlike the Sun, releases deadly radiation that is capable of reducing planets to rubble or destroying life. It cannot be seen by the naked eye as it exists on an unknown spectrum of light.