One of the most mind-boggling megastructures yet proposed, this was the central feature in Stephen Baxter's novel Ring. Whereas previously discussed structures were built on the scale of planets and stars, the Xeelee Ring was built on a scale dwarfing galaxies. This is ironic, as the "material" that made up the Ring is far too small to be seen by the naked eye: a cosmic string.
A cosmic string is a flaw in spacetime stretched into a line or loop many light years long. It shares some features with the singularities found at the hearts of black holes, but is a different kind of phenomenon. A single inch of such a string would weigh ten million billion tons on the surface of the Earth. Remnants of the Big Bang, the gravitational influence of cosmic strings is theorized to be one of the primary factors responsible not only for the formation of the first galaxies, but how they came to be distributed throughout space in large "filaments" surrounding gargantuan cosmic voids.
The physics of cosmic string are very complex, involving conical-shaped spacetime curves and other exotica. Links below lead to sites with much more extensive explanations of these phenomena.
One feature they share with singularities is their ability to severely warp space/time in their immediate vicinity, literally "dragging" the fabric of the universe behind it in a kind of wake as it spins. This has a number of odd effects, including gravity lensing and the use of the String to theoretically travel forward or backward through time.
In the novel, the alien Xeelee were losing a universe-wide war against dark matter beings for control of the cosmos. So they retreated through an "escape hatch" from the universe—created by ripping open a spinning naked singularity with a 10 million light year wide looping spiral of cosmic string spinning at near light speed. This wormhole-like "escape hatch" had a mass of several galactic clusters and measured some 300 light years across. Ten million light-years in diameter, the Ring's purpose was to rip away the singularity's event horizon to allow easy access in and out of the portal. The Xeelee Ring was so huge that the Xeelee's enemies threw entire galaxies at it to try and disrupt it.
To truly understand the scale of the Ring, the crew of the refugee human ship investigating it realized that the Ring is the source of the Great Attractor, one of the great mysteries of modern real-life astronomy. The Great Attractor is an unknown force that is pulling galaxies from hundreds of millions of light years in all directions toward some unknown concentration of mass in the direction of the constellation of Virgo. In the novel, the Xeelee Ring is revealed to be the Great Attractor, so massive (equivalent to some ten thousand galaxies) that it actually counter-acted the expansion of the universe across a significant portion of the cosmos.
It's never clear if the Xeelee simply gathered the cosmic string from around the universe or found some way of manufacturing it through some insanely advanced process. Either way, however, this represents resources and technology of a truly god-like level.
Ring by Stephen Baxter
On The Web
On Cosmic Stringshttp://www.damtp.cam.ac.uk/user/gr/public/cs_home.html http://www.pbs.org/wgbh/aso/databank/entries/dp76st.html http://www.gi.alaska.edu/ScienceForum/ASF10/1089.html http://theory.physics.unige.ch/~ringeval/strings.html http://www.centauri-dreams.org/?p=114