The mind-blowing answer comes from a theory describing the birth of the universe in the first instant of time.
The universe has long captivated us with its immense scales of distance and time.
How far does it stretch? Where does it end... and what lies beyond its star fields... and streams of galaxies extending as far as telescopes can see?
These questions are beginning to yield to a series of extraordinary new lines of investigation... and technologies that are letting us to peer into the most distant realms of the cosmos...
But also at the behavior of matter and energy on the smallest of scales.
Remarkably, our growing understanding of this kingdom of the ultra-tiny, inside the nuclei of atoms, permits us to glimpse the largest vistas of space and time.
In ancient times, most observers saw the stars as a sphere surrounding the earth, often the home of deities.
The Greeks were the first to see celestial events as phenomena, subject to human investigation... rather than the fickle whims of the Gods.
One sky-watcher, for example, suggested that meteors are made of materials found on Earth... and might have even come from the Earth.
Those early astronomers built the foundations of modern science. But they would be shocked to see the discoveries made by their counterparts today.
The stars and planets that once harbored the gods are now seen as infinitesimal parts of a vast scaffolding of matter and energy extending far out into space.
Just how far... began to emerge in the 1920s.
Working at the huge new 100-inch Hooker Telescope on California's Mt. Wilson,
astronomer Edwin Hubble, along with his assistant named Milt Humason, analyzed the light of fuzzy patches of sky... known then as nebulae.
They showed that these were actually distant galaxies far beyond our own.
Hubble and Humason discovered that most of them are moving away from us. The farther out they looked, the faster they were receding.
This fact, now known as Hubble's law, suggests that there must have been a time when the matter in all these galaxies was together in one place.
That time... when our universe sprung forth... has come to be called the Big Bang.
How large the cosmos has gotten since then depends on how long its been growing... and its expansion rate.
Recent precision measurements gathered by the Hubble space telescope and other instruments have brought a consensus...
That the universe dates back 13.7 billion years.
Its radius, then, is the distance a beam of light would have traveled in that time ... 13.7 billion light years.
That works out to about 1.3 quadrillion kilometers.
In fact, it's even bigger.... Much bigger. How it got so large, so fast, was until recently a deep mystery.
That the universe could expand had been predicted back in 1917 by Albert Einstein, except that Einstein himself didn't believe it...
until he saw Hubble and Humason's evidence.
Einstein's general theory of relativity suggested that galaxies could be moving apart because space itself is expanding.
So when a photon gets blasted out from a distant star, it moves through a cosmic landscape that is getting larger and larger, increasing the distance it must travel to reach us.
In 1995, the orbiting telescope named for Edwin Hubble began to take the measure of the universe... by looking for the most distant galaxies it could see.
Taking the expansion of the universe into account, the space telescope found galaxies that are now almost 46 billion light years away from us in each direction... and almost 92 billion light years from each other.
And that would be the whole universe... according to a straightforward model of the big bang.
But remarkably, that might be a mere speck within the universe as a whole, according to a dramatic new theory that describes the origins of the cosmos.
It's based on the discovery that energy is constantly welling up from the vacuum of space in the form of particles of opposite charge... matter and anti-matter.