I'll take a shot at a quick summary, with the caveat that it's a very complicated subject and my own understanding is far from certain.

My most recent input on cosmology comes from Leonard Susskind's "Cosmic Landscape", which I recommend for its unusually clear explanations. The cosmic landscape, according to Susskind, represents the concept of the "multiverse" as a multidimensional landscape in which the various parameters presently understood to control how a universe develops result in 10500 possible universes. (To put that number in perspective, it's estimated that there are 10300 elementary particles in this universe.)

In some sense all 10500 universes exist, but many of them represent a combination of parameters that won't form anything we'd define as a universe. A unverse's position on the landscape is a function of the energy level that results from its present characteristics, and the cosmic landscape is like a (multidimensional) terrestrial landscape composed of peaks and valleys of energy levels. Here's the kicker: A universe can always fall from a higher to a lower energy level if something happens to perturb it even slightly--some quantum fluctuation changes a parameter just enough to start a universe rolling off a high spot into a lower spot.

There's an anomaly in present theories of the history of our own universe: Its expansion started out at one rate, then some short time into the process, it accelerated. The cosmic landscape theory provides an explanation: Our universe was rolling down a shallow slope at first, then it rolled over an edge and started rolling more rapidly, until it settled into a local valley.

Notice the big implication: Our universe existed, in a way, prior to the Big Bang; it just wasn't a viable universe. Once some random quantum fluctuation perturbed it, it started rolling, and that was the Big Bang.

A very important spinoff of this work is a satisfactory explanation of the Anthropic Principle: Why is the universe so well-suited to us? Many people see in that question an argument for a Creator--"this couldn't have come about naturally, somebody (the Joker) had to stack the deck for us". In the view of cosmologists, there's a very simple and very natural explanation: The Law of Very Large Numbers. If you have 10500 possible universes, it becomes a virtual certainty that there's a universe just right for us, and here we are. No mystery at all, just simple statistics.

This is just one theory, but it seems to be a leading theory that many cosmologists think is credible. It grew out of string and brane theory, and represents at least an attempt to "look behind the curtain" and imagine what went on "before" and what goes on "around" our own universe.

More recently I read "Programming the Universe" by Seth Lloyd, and it provides an intriguing explanation for the existence of matter and energy: The Universe is a quantum computer, and what it computes is itself...that is, its computational results are mass and energy.

There's a whole lot of dense material making the case for quantum computation, and I really need to skip over it to get to the payoff. Where the cosmic landscape theory leaves off, at the Big Bang, Lloyd believes that quantum computing took over.

The way the Universe computes, according to Lloyd, is reminiscent of biological evolution. Quantum fluctuations of spacetime--miniscule bits of energy and matter popping in and out of existence-- happen constantly, at random, at the very fine scale of Planck's Constant (something like 10-43 meters). Left to their own devices, these fluctuations cancel out and would have no effect on the Universe. However, these fluctuations are acted upon by the laws of the universe to select fluctuations that are "viable" according to the laws, and "decohere" the others. The clearest example is the way that quantum mass fluctuations are selected by the law of gravity to promote clumping of matter, a process that snowballs to eventually produce galaxies and stars and planets and us etc. The universe right after the Big Bang was smooth and homogenous, but the quantum fluctuations were enough to introduce just enough irregularity to allow the universe to evolve into the form we know it today.

Why did the universe expand? Because at the instant of the Big Bang conditions allowed one bit of quantum computation to take place. That first cycle made "room" for a second bit at the next cycle (one Planck's time unit later), four bits in the third cycle, eight, 16, 32...bits are physical in Lloyd's view, and occupy an infinitesmal region of space, and the more bits you have, the bigger a universe you have.

That's as quick a summary as I can manage on so complicated a subject. I recommend Susskind's book very highly, though I can't recommend Lloyd's quite so enthusiastically. The latter's explanations aren't as clear and rely more on assertion than actual explanation. But Susskind's way of explaining things led me to several "ah ha!" moments.