"Ex nihilo, omni..." -- "Out of nothing, everything..."
A long time ago, in a universe infinitely far from and yet in some ways infinitely close to our own Universe, a small portion of completely empty space seethed with a maelstrom of sub-atomic fury. At the macroscopic or "real-world" level where we can directly perceive things, and in the microscopic world which we can probe with sensitive measuring tools, absolutely nothing existed or appeared to be happening in that empty space. But in a sub-sub-sub-microscopic world far beyond the limits of observation, energy and matter were being created and destroyed, coming into being and flashing out of existence at an infinite rate.
Most of this frenzied activity involved amounts of energy and types of particles so small and impermanent that even had they existed for a measurable time, they would have been too small or too briefly in existence for any observer to be sure that they had actually observed something, or whether any "observations" were simply part of the "noise" or random error that accompanies any observation of things which are far smaller than the sub-atomic world. But for one incredibly brief moment -- about a billionth of a trillionth of a trillionth of a trillionth of a second -- a cosmic glitch occurred, one in which the energy of the unobservable existence and nonexistence which ceaselessly roiled this space grew by trillions of trillions of trillions of times, many times over. And in that instant a new cosmos was created, a new piece of space and time with a mass and energy far greater than that of all the galaxies and clusters of galaxies which fill our universe, and in the same instant, was destroyed.
More accurately, it ceased to exist in the universe which had given it birth in the same instant in which it was created. But in that instant the space and time which had come into being grew and grew, expanding at a virtually infinite rate in all directions, and yet in no real "direction" at all, since none of the space which was being created was taking up any previously existing space, either in the universe which created it or in any other universe, but was simply coming into existence where nothing -- not even empty space -- had existed before.
As strange as it seems, this is the way in which we believe new universes are created, and in particular how our own Universe came to be -- as an infinitely brief blink of an imaginary eye in a previously existing universe which was unseen by and undetectable by anyone who might have happened to be there. But in the newly created space-time of our own Universe, if there had been anyone to observe who would not have been instantly annihilated by the infinite temperatures and energies of the stuff which filled that nascent Universe, everything would have been just as real as the everyday world which surrounds us.
The infinitely powerful fireworks display which marked our Universe's beginning is long over, but looking out into the vastness of space we look back in time and can study the Universe not only as it is, but also as it was; and as our studies open that door to the past, we come to an understanding of the way in which we and everything that surrounds us to the furthest limits of the Universe came to be, grew, aged and evolved, and what the limitless future holds in store.
On the left, the creation of our Universe, as seen in the space-time in which it originated. For an instant too brief to notice (despite being slowed down in the animation by several trillion trillion trillion times), things aren't quite as usual. (At the start, the left image is completely blank. As the animation changes on the right, a single pixel blinks on and off near the center of the left image.)
On the right, the creation of our Universe as seen in our space-time. Prior to the beginning of the Universe, there was nothing -- no space, time, energy, or matter. Then, in the same instant that "something" happened and yet didn't exactly happen in the original space-time, a new space-time of infinite density and energy was created and instantly expanded to nearly infinite size.