In their view of the universe the complexities of an inflating universe after a Big Bang are replaced by a universe that was already large. flat, and uniform with dark energy as the effect of the other universe constantly leaking gravity into our own and driving its acceleration. According to this theory, the Big Bang was not the beginning of time but the bridge to a past filled with endlessly repeating cycles of evolution, each accompanied by the creation of new matter and the formation of new galaxies, stars, and planets.
Turok and Steinhardt were inspired by a lecture given by Burt Ovrut who imagined two branes, universes like ours, separated by a tiny gap as tiny as 10-32 meters. There would be no communictaion between the two universes except for our parallel sister universe's gravitational pull, which could cross the tiny gap.
Orvut's theory could explain the effect of dark matter where areas of the universe are heavier than they should be given everything that's present. With their theory, the nagging problems surrounding the Big Bang (beginning from what, and caused how?) are replaced by an eternal cosmic cycle where dark energy is no longer a mysterious unknown quantity, but rather the very extra gravitational force that drives the universe to universe (brane-brane) interaction.
Source: http://endlessuniverse.net/ http://www.damtp.cam.ac.uk/user/ngt1000/