The answer - my answer, anyway - is 'imagination.' It too is pretty simple, and yet even the most complex of theories are rooted in seemingly simple ideas.
Imagination is what has brought us to this point of existence today: the computer you're looking at, the cable that connects it to remote servers, even the chair you are sitting on! All of it is the product of imagination. It is the energy source that has driven human ascent, has shaped the planet itself, and as we reach out to distant stars and galaxies, it will determine where we go in the future - for better or for worse. Consider for a moment: is every idea a good idea? If imagination is what has shaped our existence, look around you and ask yourself this: is this as good as it gets? Is this the best we could have done?
Imagination is a powerful source, and it's open to everybody too - open source, if you like that - but it is both creative and destructive at the same time. The same technology that runs the GPS in my car is used in the Star Wars weapons program to knock down enemy missiles. Same technology but applied very differently. Perhaps then imagination is agnostic, and it is people that are the determining factor. At least, that is, until we unleash technology that can think for itself. And then, how will it use the power of imagination? For better or for worse?
The concept of a technological singularity may be familiar to you: the idea that technology will invent subsequent versions of itself, becoming so complex and powerful that humans will be unable to comprehend it. It will cease to exist as far as we are concerned (and vice versa quite frankly). In my book, I explore the theory that events leading up to the singularity horizon will be visible to humans, they will be time-based events; in fact, they will be how we measure our own rather self-absorbed existence. As the singularity approaches, however, events will exponentially outstrip time until time runs out. And when you run out of time, life as you know it runs out too.
But what is left, floating out in the ether will be a spark, a spark of imagination. An idea to create a new world, or an entire new universe of worlds, an idea that is irrepressible and must find a way to come forth into being. That is the power of imagination.