The universe is made out of fuel.

Something like 98% of the observable mass of the universe is hydrogen. Hydrogen is a near-perfect fuel; even it’s disadvantages stem from it’s advantages. (For example, being so atomically simple means that it is usable in a host of high-energy reactions; that same simplicity makes it a little bit hard to store over long periods.)

Locally, in our own solar system, sources of hydrogen are slightly less accessible. Most of it is locked up in the Sun, or in the atmospheres of the gas giants. To make it on Earth is expensive in terms of energy; finding it in space is much easier in energy costs but much harder in technological costs. We know how to build hydrogen cracking plants today to generate it from sea water; you can build a model plant in your backyard with a knowledge of high-school chemistry. But we have no clue about how to get at the near-infinite quantities of refinable material in the atmospheres of Jupiter and Saturn. We can get there, we can get back; space elevators have even solved (in principle) the problem of moving the stuff down here to Earth where we can put it in our cars. But it’s out there, and the human race is pretty good at finding ways to get to the next apple on the branch.

In addition, fuels less exotic than hydrogen are in abundance in the solar system. Robert Zubrin, who is now making waves with his revolutionary ideas about fueling cars, previously made waves with his revolutionary ideas for getting to Mars and back. The chemical composition of Martian soil makes it a decent source of rocket fuel; all that you have to provide is oxidizer and some way of mining and processing. But mining and processing are, if not easy, at least tasks that we know how to accomplish. Mars could be the rocket fuel depot of the future solar system.

Even more convenient, the surface of Titan turns out to be covered with what is essentially oil. Hydrocarbon lakes of enormous extent are literally sitting on the surface, waiting to be pumped away. God only knows what’s beneath the surface. Titan may be the next Texas.

Getting this stuff back to Earth isn’t easy, or free - but it isn’t as expensive as many assume. The research will be done, the rockets and space elevators built, and the fuel economy will continue. Whether you believe in God or random chance, we have a blessing of convenient fuels available for us and there is no end in sight.