"The Realm of Darkness, the true beauty of the Universe. This is what I want all of existence to be like...a place of pure darkness where I can rule supreme!"
There is very little ecology in The Realm of Darkness, some of the only living things in this realm are not even considered living and are referred to as shades of death, beings that are neither living nor dead. The only other living being in this realm is it's creator, The Dark Master who made this realm himself.
The climate is very cold and has the feeling of pure evil in it's atmosphere. The Realm of Darkness has different densities of gravity such as The Citadel of The Dark Master, which has a gravity very similar to Earth's but the rest of the realm has an almost moon like gravity.
The Realm of Darkness was created eons ago by The Dark Master as a pocket dimension so he could live outside the Universe in his own kingdom of darkness but he also created it so he could monitor the Universe for any opportunities that could help him achive his goals.
In Sonic: Beyond, The Realm of Darkness serves as the final location in Part 1 and is presented to the heroes as a place of pure evil.
Sonic: Age of Apocalypse
In Sonic: Age of Apocalypse, the heroes returned to The Realm of Darkness in order to try and gain access to other Universes through The Realm of Darkness, due to it being placed outside the Universe.