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When we look out into space, we're looking back in time; the light from a galaxy a billion light-years away, for instance, will take a billion years to reach us. It's an amazing thing. The history is there for us to see. It's not mushed up like the geologic record of Earth. You can just see it exactly as it was.

The Big Bang theory is the idea that if we go back early enough in the history of the universe - and we can do this, of course, by looking at starlight coming to us from billions of years ago - we will see a very hot and dense period where the universe was much smaller, denser, and hotter.

People have contemplated the origin and evolution of the universe since before the time of Aristotle. Very recently, the era of speculation has given way to a time of science.

A full understanding of what happens in our everyday lives needs to take into account what happened at the Big Bang. And not only is that intrinsically interesting and just kind of cool to think about, but it's also a mystery that is not given much attention by working scientists; it's a little bit underappreciated.

Scientists - who prefer explanations subject to laboratory tests - figure that everything we see today was as inevitable as wrinkles, once the Big Bang established physics. Stars and planets were cooked up as huge clouds of matter collapsed and coalesced.

And what we know, or think we know, about the universe of space and time is changing very quickly.

We see past time in a telescope and present time in a microscope. Hence the apparent enormities of the present.

We can trace things back to the earlier stages of the Big Bang, but we still don't know what banged and why it banged. That's a challenge for 21st-century science.

The cosmos is three times as old as Earth. During most of creation's 14 billion year history, our solar system wasn't around. Nonetheless, the early universe still had the right stuff for life, and contained worlds that were just as suitable for spawning biology and intelligence as our own.