A hypothetical four-dimensional space in which the ordinary 3-dimensional space of the known universe exists. In M-Theory, 3-space is called a 3-brane, a 3-dimensional analogue of a string. Our universe is a boundary surface of the 4-brane called bulk space.

A new theory called the ekpyrotic universe model has been proposed as an alternative to cosmic inflation. This says the Big Bang could have happened as the result of a collision between two universes, or bulk branes, in bulk space. This caused a release of energy, the Big Bang, or the "brane storm".

Source: New Scientist, No. 2286, 14 April 2001.

More at: http://xxx.lanl.gov/abs/hep-th/0103239