Webway
The Webway is an ancient structure, older than even the Eldar, which can be described as a network of highways through the Warp, or on the skein between the Warp and realspace. The Eldar make extensive use of the Webway for travel, as Eldar souls are especially prized by Daemons, and Webway travel is less risky. Some parts of the Webway are so large that entire cities can fit in it, like the Dark Eldar city of Commorragh.
Eldar have the most widespread access to the Webway. The Golden Throne was supposed to be an Imperial access point from which the Imperium could conquer the Webway for humanity (thereby eliminating the need for Warp travel, and striking a major blow against Chaos), but Magnus the Red's ill-fated attempt to warn the God-Emperor of Mankind about the Horus Heresy wrecked that part of the device. Another human-engineered entry to the Webway, code named "Dark Glass", was constructed in another part of the galaxy, but abandoned shortly after being partly completed. Unlike the gateway on Terra, this gate was partly made in collusion with the Navigators, despite the fact that its success would render them redundant. It was used once by the White Scars Legion to make their way back to Terra and was destroyed under the strain. To make matters even more grimdark, the Eldar really have no idea how the damn things work. Despite being taught how, they don't change anything about it because that's plot development talk, and even if they knew, they wouldn't tell the Imperium. (To be fair to the Eldar, the Imperium wouldn't even think to politely ask for their help.) The Webway was created by the Old Ones, the Eldar's deadbeat absentee parents who had the gall to go extinct before explaining it to their kids. Fast forward a few dozen millennia and nobody remembers the routes anymore, and even if you do, there are pretty good odds the tunnel has been compromised by the Warp and infested with daemons or other, stranger creatures since the Fall or other major psychic events (remember Magnus the Red?).
Despite the loss of the Golden Throne, the Adeptus Mechanicus had shown that use of the Webway is fairly simple for humans. The Forgeworld of Stygies being able to not only operate a functioning Webway Gate, but also managing to move through with an army large enough to reach Commorragh itself. Indeed, notable examples such as Vulkan's journey to Terra was recorded, and some of the forces of Chaos, such as Magnus the Red and Ahriman took advantage of the ease of movement within the Webway themselves.
The Webway is like a living thing, and is in many ways similar to the human circulatory system (in fact its paths are even called arteries, veins, and capillaries sometimes). Paths move, some are lost while new ones form, etc. And it still opens up to millions billions of locations around the galaxy. So, basically, the Dark Eldar live there, and otherwise it's like the Warp, but (slightly) more stable. And thanks to this chucklefuck, the Necrons can use it now, but their method is unreliable and they have limited access. God dammit Matt Ward! (In all fairness, this move was most likely to limit Necron growth, as using their old FTL would result in them sending fleets to invade everywhere.)