The Big Bang theory has been a tremendously successful cosmological model. But its biggest failures have been its inability to explain how the theory of general relativity, a theory of gravity, works with the theory of quantum mechanics, the theory of the atom. The Big Bang theory can not explain how the smallest part of the universe works with the largest part.
Several theories of multiple universes colliding have emerged to correct this problem. One of these is the Ekpyrotic model. This model is based on the hypothesis that there were two three-dimensional universes moving along a separate but hidden dimension. As these three-dimensional universes move along side of each other they collide, and the kinetic energy that is created from the impact is converted into the quarks, electrons, photons, and other elementary particles that are confined to move along the three dimensions that we see.
The resulting collision temperature is finite, in contrast to the hot big bang, and this phase begins without a singularity- central to the Big Bang theory. The universe is homogeneous(looks the same everywhere) because the collision and initiation of the collision phase occurs simultaneously everywhere. The geometry for the two worlds that meet is flat, so the collision produces a flat big bang universe. So we turn to Einstein's equations to understand this phenomenon. His equations say that the total energy density found in the Universe will be equal to its critical density. So one way of understanding this is that one of the overabundant particles found in the early Big Bang universe, the massive magnetic monopoles, are not produced at all in this collision model because the temperature after the collision is far too small to produce any of these massive particles. The Big Bang theory produces particles that can not be found, or energies that are unexplained. Collision theory avoids those problems.
Source: A Brief Introduction to the Ekpyrotic Universe